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

[Vlad Serebrennikov via cfe-commits]

https://github.com/Endilll commented:

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

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