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

[Jannick Kremer via cfe-commits]

DeinAlptraum wrote:

Have we documented exact requirements for this anywhere?
Also not sure what the point of stability would be here: `clang_File_isEqual` returns `True` for all comparisons of in-memory file, which just makes this useless in practice. Any code that relies on this would be wrong to begin with...

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