[clang-tidy] Extract string header from redundant-string-cstr checker

In preparation for using the implementation of basic_string and
basic_string_view from redundant-string-cstr.cpp in other checks, let's
extract it to a header.

Using the existing <string.h> to provide size_t, using that to define
size_type and using it rather than the previous "size" type makes it
easier to provide the size() method later and makes the implementation
closer to the standard.

Reviewed By: carlosgalvezp

Differential Revision: https://reviews.llvm.org/D144216

clang-tools-extra/test/clang-tidy/checkers/Inputs/Headers/string

@@ -0,0 +1,74 @@
+#ifndef _STRING_
+#define _STRING_
+
+// For size_t
+#include <string.h>
+
+typedef unsigned __INT16_TYPE__ char16;
+typedef unsigned __INT32_TYPE__ char32;
+
+namespace std {
+template <typename T>
+class allocator {};
+template <typename T>
+class char_traits {};
+template <typename C, typename T, typename A>
+struct basic_string {
+  typedef size_t size_type;
+  typedef basic_string<C, T, A> _Type;
+  basic_string();
+  basic_string(const C *p, const A &a = A());
+
+  ~basic_string();
+
+  const C *c_str() const;
+  const C *data() const;
+
+  _Type& append(const C *s);
+  _Type& append(const C *s, size_type n);
+  _Type& assign(const C *s);
+  _Type& assign(const C *s, size_type n);
+
+  int compare(const _Type&) const;
+  int compare(const C* s) const;
+  int compare(size_type pos, size_type len, const _Type&) const;
+  int compare(size_type pos, size_type len, const C* s) const;
+
+  size_type find(const _Type& str, size_type pos = 0) const;
+  size_type find(const C* s, size_type pos = 0) const;
+  size_type find(const C* s, size_type pos, size_type n) const;
+
+  _Type& insert(size_type pos, const _Type& str);
+  _Type& insert(size_type pos, const C* s);
+  _Type& insert(size_type pos, const C* s, size_type n);
+
+  _Type& operator+=(const _Type& str);
+  _Type& operator+=(const C* s);
+  _Type& operator=(const _Type& str);
+  _Type& operator=(const C* s);
+};
+
+typedef basic_string<char, std::char_traits<char>, std::allocator<char>> string;
+typedef basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t>> wstring;
+typedef basic_string<char16, std::char_traits<char16>, std::allocator<char16>> u16string;
+typedef basic_string<char32, std::char_traits<char32>, std::allocator<char32>> u32string;
+
+template <typename C, typename T>
+struct basic_string_view {
+  basic_string_view(const C* s);
+};
+typedef basic_string_view<char, std::char_traits<char>> string_view;
+typedef basic_string_view<wchar_t, std::char_traits<wchar_t>> wstring_view;
+typedef basic_string_view<char16, std::char_traits<char16>> u16string_view;
+typedef basic_string_view<char32, std::char_traits<char32>> u32string_view;
+
+std::string operator+(const std::string&, const std::string&);
+std::string operator+(const std::string&, const char*);
+std::string operator+(const char*, const std::string&);
+
+bool operator==(const std::string&, const std::string&);
+bool operator==(const std::string&, const char*);
+bool operator==(const char*, const std::string&);
+}
+
+#endif // _STRING_

clang-tools-extra/test/clang-tidy/checkers/readability/redundant-string-cstr.cpp

@@ -1,71 +1,5 @@
-// RUN: %check_clang_tidy %s readability-redundant-string-cstr %t
-
-typedef unsigned __INT16_TYPE__ char16;
-typedef unsigned __INT32_TYPE__ char32;
-typedef __SIZE_TYPE__ size;
-
-namespace std {
-template <typename T>
-class allocator {};
-template <typename T>
-class char_traits {};
-template <typename C, typename T, typename A>
-struct basic_string {
-  typedef basic_string<C, T, A> _Type;
-  basic_string();
-  basic_string(const C *p, const A &a = A());
-
-  ~basic_string();
-
-  const C *c_str() const;
-  const C *data() const;
-
-  _Type& append(const C *s);
-  _Type& append(const C *s, size n);
-  _Type& assign(const C *s);
-  _Type& assign(const C *s, size n);
-
-  int compare(const _Type&) const;
-  int compare(const C* s) const;
-  int compare(size pos, size len, const _Type&) const;
-  int compare(size pos, size len, const C* s) const;
-
-  size find(const _Type& str, size pos = 0) const;
-  size find(const C* s, size pos = 0) const;
-  size find(const C* s, size pos, size n) const;
-
-  _Type& insert(size pos, const _Type& str);
-  _Type& insert(size pos, const C* s);
-  _Type& insert(size pos, const C* s, size n);
-
-  _Type& operator+=(const _Type& str);
-  _Type& operator+=(const C* s);
-  _Type& operator=(const _Type& str);
-  _Type& operator=(const C* s);
-};
-
-typedef basic_string<char, std::char_traits<char>, std::allocator<char>> string;
-typedef basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t>> wstring;
-typedef basic_string<char16, std::char_traits<char16>, std::allocator<char16>> u16string;
-typedef basic_string<char32, std::char_traits<char32>, std::allocator<char32>> u32string;
-
-template <typename C, typename T>
-struct basic_string_view {
-  basic_string_view(const C* s);
-};
-typedef basic_string_view<char, std::char_traits<char>> string_view;
-typedef basic_string_view<wchar_t, std::char_traits<wchar_t>> wstring_view;
-typedef basic_string_view<char16, std::char_traits<char16>> u16string_view;
-typedef basic_string_view<char32, std::char_traits<char32>> u32string_view;
-}
-
-std::string operator+(const std::string&, const std::string&);
-std::string operator+(const std::string&, const char*);
-std::string operator+(const char*, const std::string&);
-
-bool operator==(const std::string&, const std::string&);
-bool operator==(const std::string&, const char*);
-bool operator==(const char*, const std::string&);
+// RUN: %check_clang_tidy %s readability-redundant-string-cstr %t -- -- -isystem %clang_tidy_headers
+#include <string>
 
 namespace llvm {
 struct StringRef {