[clang] Initialize the file system explicitly (#158381)

This PR is a part of the effort to make the VFS used in the compiler
more explicit and consistent.

Instead of creating the VFS deep within the compiler (in
`CompilerInstance::createFileManager()`), clients are now required to
explicitly call `CompilerInstance::createVirtualFileSystem()` and
provide the base VFS from the outside.

This PR also helps in breaking up the dependency cycle where creating a
properly configured `DiagnosticsEngine` requires a properly configured
VFS, but creating properly configuring a VFS requires the
`DiagnosticsEngine`.

Both `CompilerInstance::create{FileManager,Diagnostics}()` now just use
the VFS already in `CompilerInstance` instead of taking one as a
parameter, making the VFS consistent across the instance sub-object.

1 files changed, 5 insertions, 3 deletions

diff --git a/clang/lib/Frontend/ASTUnit.cpp b/clang/lib/Frontend/ASTUnit.cpp
index 03b08cd..8b35af1 100644
--- a/clang/lib/Frontend/ASTUnit.cpp
+++ b/clang/lib/Frontend/ASTUnit.cpp
@@ -1189,10 +1189,12 @@ bool ASTUnit::Parse(std::shared_ptr<PCHContainerOperations> PCHContainerOps,
   // Ensure that Clang has a FileManager with the right VFS, which may have
   // changed above in AddImplicitPreamble.  If VFS is nullptr, rely on
   // createFileManager to create one.
-  if (VFS && FileMgr && &FileMgr->getVirtualFileSystem() == VFS)
+  if (VFS && FileMgr && &FileMgr->getVirtualFileSystem() == VFS) {
+    Clang->setVirtualFileSystem(std::move(VFS));
     Clang->setFileManager(FileMgr);
-  else {
-    Clang->createFileManager(std::move(VFS));
+  } else {
+    Clang->setVirtualFileSystem(std::move(VFS));
+    Clang->createFileManager();
     FileMgr = Clang->getFileManagerPtr();
   }