[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