[clang] [libclang/C++] Fix clang_File_isEqual for in-memory files (PR #135773)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> LGTM, but how this fares against our stability guarantees for libclang APIs? Do we need `clang_File_isEqual2`?

I think this is a bug fix and so it meets our requirements. The ABI is stable, but we can fix logic bugs.

https://github.com/llvm/llvm-project/pull/135773