[clang] [clang] Introduce '-fint64-is-long-long' (PR #234780)

[Hubert Tong via cfe-commits]

hubert-reinterpretcast wrote:

> Do you have a suggestion on where and what I should test with regard to those? I am failing to find some existing tests for those macros.

I think, for the compiler, what needs to be covered in that dimension are the `__[U]INT64_FMT*__` macros. These are separately covered in the various `clang/test/Preprocessor/init*.c` files.

https://github.com/llvm/llvm-project/pull/234780