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

[Neil Henning via cfe-commits]

================
@@ -1115,13 +1115,13 @@ void ASTWriter::WriteBlockInfoBlock() {
 }
 
 /// Prepares a path for being written to an AST file by converting it
-/// to an absolute path and removing nested './'s.
+/// to an absolute path and removing nested './'s and '../'s.
 ///
 /// \return \c true if the path was changed.
 static bool cleanPathForOutput(FileManager &FileMgr,
                                SmallVectorImpl<char> &Path) {
   bool Changed = FileMgr.makeAbsolutePath(Path);
-  return Changed | llvm::sys::path::remove_dots(Path);
+  return Changed | llvm::sys::path::remove_dots(Path, true);
----------------
sheredom wrote:

Added https://github.com/llvm/llvm-project/pull/106577/files#diff-3056397628ccde0c171064fdbedcdf91ff1ae3691f8dd1133150fc2cebf92277R11 which actually uncovered a bug in the impl and fixed that too (was reusing `PathStr` wrongly).

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