[clang] [clang][modules] Minor improvements to diagnosing `out of date` errors (PR #136612)

[Cyndy Ishida via cfe-commits]

cyndyishida wrote:

> What would happen if both A-1.pcm and A-2.pcm are built for the same files (e.g., from Sysroot) but have different file names? Not saying it is a valid use case to support, just curious what would happen.

I tried this, where basically `A-1.pcm` is built again but with `SomethingRandom-1.pcm` and passed that to the final client (B still depends on A-1.pcm). The compilation passed, so I think that means clang doesn't care about the name of the file. 

```
/Users/cishida/Projects/llvm/monorepo/llvm-project/clang/test/Modules/invalid-module-dep.c:37:2: remark: importing module 'B' from '/Users/cishida/Builds/llvm-build/tools/clang/test/Modules/Output/invalid-module-dep.c.tmp/B-1.pcm' [-Rmodule-import]
   37 | #include <B/B.h>
      |  ^
/Users/cishida/Projects/llvm/monorepo/llvm-project/clang/test/Modules/invalid-module-dep.c:37:2: remark: importing module 'A' into 'B' from '/Users/cishida/Builds/llvm-build/tools/clang/test/Modules/Output/invalid-module-dep.c.tmp/SomethingRandom-1.pcm' [-Rmodule-import]
```

https://github.com/llvm/llvm-project/pull/136612