[clang] Support '-fmodule-file-home-is-cwd' for C++ modules. (PR #135147)

[Michael Park via cfe-commits]

mpark wrote:

Ah, interesting. It looks like indeed, this doesn't apply to C++ named modules.
```
      // We don't want to hard code the information about imported modules
      // in the C++20 named modules.
```
https://github.com/llvm/llvm-project/blob/main/clang/lib/Serialization/ASTWriter.cpp#L1593-L1608

https://github.com/llvm/llvm-project/pull/135147