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

[Jannick Kremer via cfe-commits]

DeinAlptraum wrote:

@AaronBallman this is the fix PR for the bug we discussed at https://github.com/llvm/llvm-project/pull/130383#discussion_r2014205462

The first test is for completeness' sake, the second one (with in-memory files) fails without the fix

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