[clang][Sema] Suggest static_cast in C++ code

This patch changes the -Wformat diagnostic to suggest static_cast over
a C-style cast for {,Objective}C++ when recommending the argument be
casted rather than changing the format string.

Before:
```
clang/test/FixIt/format.mm:11:16: warning: format specifies type 'unichar' (aka 'unsigned short') but the argument has type 'wchar_t' [-Wformat]
   11 |   NSLog(@"%C", wchar_data);  // expected-warning{{format specifies type 'unichar' (aka 'unsigned short') but the argument has type 'wchar_t'}}
      |           ~~   ^~~~~~~~~~
      |                (unsigned short)
```
After:
```
clang/test/FixIt/format.mm:11:16: warning: format specifies type 'unichar' (aka 'unsigned short') but the argument has type 'wchar_t' [-Wformat]
   11 |   NSLog(@"%C", wchar_data);  // expected-warning{{format specifies type 'unichar' (aka 'unsigned short') but the argument has type 'wchar_t'}}
      |           ~~   ^~~~~~~~~~
      |                static_cast<unsigned short>( )
```

Differential Revision: https://reviews.llvm.org/D153622

clang/docs/ReleaseNotes.rst

@@ -415,6 +415,8 @@ Improvements to Clang's diagnostics
     source:1:6: note: candidate function not viable: no known conversion from 'const char[4]' to 'int' for 2nd argument
     void func(int aa, int bb);
          ^            ~~~~~~
+- ``-Wformat`` cast fix-its will now suggest ``static_cast`` instead of C-style casts
+  for C++ code.
 
 Bug Fixes in This Version
 -------------------------

clang/lib/Sema/SemaChecking.cpp

@@ -11184,9 +11184,9 @@ CheckPrintfHandler::checkFormatExpr(const analyze_printf::PrintfSpecifier &FS,
       // if necessary).
       SmallString<16> CastBuf;
       llvm::raw_svector_ostream CastFix(CastBuf);
-      CastFix << "(";
+      CastFix << (S.LangOpts.CPlusPlus ? "static_cast<" : "(");
       IntendedTy.print(CastFix, S.Context.getPrintingPolicy());
-      CastFix << ")";
+      CastFix << (S.LangOpts.CPlusPlus ? ">" : ")");
 
       SmallVector<FixItHint,4> Hints;
       if (!AT.matchesType(S.Context, IntendedTy) || ShouldNotPrintDirectly)
@@ -11197,7 +11197,7 @@ CheckPrintfHandler::checkFormatExpr(const analyze_printf::PrintfSpecifier &FS,
         SourceRange CastRange(CCast->getLParenLoc(), CCast->getRParenLoc());
         Hints.push_back(FixItHint::CreateReplacement(CastRange, CastFix.str()));
 
-      } else if (!requiresParensToAddCast(E)) {
+      } else if (!requiresParensToAddCast(E) && !S.LangOpts.CPlusPlus) {
         // If the expression has high enough precedence,
         // just write the C-style cast.
         Hints.push_back(

clang/test/FixIt/format.mm

@@ -9,22 +9,22 @@ void test_percent_C() {
 
   const wchar_t wchar_data = L'a';
   NSLog(@"%C", wchar_data);  // expected-warning{{format specifies type 'unichar' (aka 'unsigned short') but the argument has type 'wchar_t'}}
-  // CHECK: fix-it:"{{.*}}":{[[@LINE-1]]:16-[[@LINE-1]]:16}:"(unsigned short)"
+  // CHECK: fix-it:"{{.*}}":{[[@LINE-1]]:16-[[@LINE-1]]:16}:"static_cast<unsigned short>(
 
   NSLog(@"%C", 0x260300);  // expected-warning{{format specifies type 'unichar' (aka 'unsigned short') but the argument has type 'int'}}
   // CHECK-NOT: fix-it:"{{.*}}":{[[@LINE-1]]:11-[[@LINE-1]]:13}:"%d"
-  // CHECK: fix-it:"{{.*}}":{[[@LINE-2]]:16-[[@LINE-2]]:16}:"(unsigned short)"
+  // CHECK: fix-it:"{{.*}}":{[[@LINE-2]]:16-[[@LINE-2]]:16}:"static_cast<unsigned short>(
 
   typedef unsigned short unichar;
 
   NSLog(@"%C", wchar_data);  // expected-warning{{format specifies type 'unichar' (aka 'unsigned short') but the argument has type 'wchar_t'}}
-  // CHECK: fix-it:"{{.*}}":{[[@LINE-1]]:16-[[@LINE-1]]:16}:"(unichar)"
+  // CHECK: fix-it:"{{.*}}":{[[@LINE-1]]:16-[[@LINE-1]]:16}:"static_cast<unichar>(
 
   NSLog(@"%C", 0x260300);  // expected-warning{{format specifies type 'unichar' (aka 'unsigned short') but the argument has type 'int'}}
   // CHECK-NOT: fix-it:"{{.*}}":{[[@LINE-1]]:11-[[@LINE-1]]:13}:"%d"
-  // CHECK: fix-it:"{{.*}}":{[[@LINE-2]]:16-[[@LINE-2]]:16}:"(unichar)"
+  // CHECK: fix-it:"{{.*}}":{[[@LINE-2]]:16-[[@LINE-2]]:16}:"static_cast<unichar>(
 
   NSLog(@"%C", 0.0); // expected-warning{{format specifies type 'unichar' (aka 'unsigned short') but the argument has type 'double'}}
   // CHECK: fix-it:"{{.*}}":{[[@LINE-1]]:11-[[@LINE-1]]:13}:"%f"
-  // CHECK-NOT: fix-it:"{{.*}}":{[[@LINE-2]]:16-[[@LINE-2]]:16}:"(unichar)"
+  // CHECK-NOT: fix-it:"{{.*}}":{[[@LINE-2]]:16-[[@LINE-2]]:16}:"static_cast<unichar>(
 }