[clang-tidy] Make readability-string-compare check use <string> header

Improve the generic <string> header by adding another constructor,
std::basic_string::empty and operator!= overload set so that it can be
used to replace the custom implementation in the
readability-string-compare check.

Depends on D145311

Reviewed By: PiotrZSL

Differential Revision: https://reviews.llvm.org/D145312

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

@@ -18,12 +18,15 @@ 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 *p, size_type count);
 
   ~basic_string();
 
   const C *c_str() const;
   const C *data() const;
 
+  bool empty() const;
+
   _Type& append(const C *s);
   _Type& append(const C *s, size_type n);
   _Type& assign(const C *s);
@@ -72,6 +75,10 @@ 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&);
+
+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/string-compare.cpp

@@ -1,25 +1,5 @@
-// RUN: %check_clang_tidy %s readability-string-compare %t
-
-namespace std {
-template <typename T>
-class allocator {};
-template <typename T>
-class char_traits {};
-template <typename C, typename T = std::char_traits<C>, typename A = std::allocator<C>>
-class basic_string {
-public:
-  basic_string();
-  basic_string(const C *, unsigned int size);
-  int compare(const basic_string<char> &str) const;
-  int compare(const C *) const;
-  int compare(int, int, const basic_string<char> &str) const;
-  bool empty();
-};
-bool operator==(const basic_string<char> &lhs, const basic_string<char> &rhs);
-bool operator!=(const basic_string<char> &lhs, const basic_string<char> &rhs);
-bool operator==(const basic_string<char> &lhs, const char *&rhs);
-typedef basic_string<char> string;
-}
+// RUN: %check_clang_tidy %s readability-string-compare %t -- -- -isystem %clang_tidy_headers
+#include <string>
 
 void func(bool b);