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

[Nathan Chancellor via cfe-commits]

nathanchance wrote:

> 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.

Thanks a lot for the assist, I have added a test for those in 085dee8ad12ed853ca70a99a9c3b864c051576a1.

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