[clang] Make PCH's respect any VFS specified. (PR #106577)

[Neil Henning via cfe-commits]

================
@@ -4772,6 +4772,23 @@ bool ASTWriter::PreparePathForOutput(SmallVectorImpl<char> &Path) {
     Changed = true;
   }
 
+  // If we are generating a normal PCH (EG. not a C++ module).
+  if (!WritingModule) {
----------------
sheredom wrote:

Note that without this guard the following tests would fail:

```
Failed Tests (2):
  Clang :: ClangScanDeps/modules-file-name-as-requested.m
  Clang :: ClangScanDeps/modules-implementation-vfs.m
```

https://github.com/llvm/llvm-project/pull/106577