[clang][modules] Name the module map files on PCM file conflict (#134475)

With implicitly-built modules, seeing something like:
```
fatal error: module 'X' is defined in both '<cache>/HASH1/X-HASH2.pcm' and '<cache>/HASH1/X-HASH3.pcm'
```
is super confusing and not actionable, because the module cache tends to
be hidden from the developer.

This PR adds a note to that diagnostic that names the module map files
the PCM files were compiled from, hopefully giving a good enough hint
for further investigation:
```
note: compiled from '<build>/X.framework/Modules/module.modulemap' and '<SDK>/X.framework/Modules/module.modulemap'
```

(I had to replace the mechanism used to convert `DiagnosticError` into
something `DiagnosticsEngine` can understand, because it seemingly did
not support notes.)

0 files changed, 0 insertions, 0 deletions