Recommit virtual file system

Previously reverted in r201755 due to causing an assertion failure.

I've removed the offending assertion, and taught the CompilerInstance to
create a default virtual file system inside createFileManager. In the
future, we should be able to reach into the CompilerInvocation to
customize this behaviour without breaking clients that don't care.

llvm-svn: 201818

clang/lib/Frontend/CompilerInstance.cpp

@@ -79,6 +79,10 @@ void CompilerInstance::setTarget(TargetInfo *Value) {
 
 void CompilerInstance::setFileManager(FileManager *Value) {
   FileMgr = Value;
+  if (Value)
+    VirtualFileSystem = Value->getVirtualFileSystem();
+  else
+    VirtualFileSystem.reset();
 }
 
 void CompilerInstance::setSourceManager(SourceManager *Value) {
@@ -197,7 +201,11 @@ CompilerInstance::createDiagnostics(DiagnosticOptions *Opts,
 // File Manager
 
 void CompilerInstance::createFileManager() {
-  FileMgr = new FileManager(getFileSystemOpts());
+  if (!hasVirtualFileSystem()) {
+    // TODO: choose the virtual file system based on the CompilerInvocation.
+    setVirtualFileSystem(vfs::getRealFileSystem());
+  }
+  FileMgr = new FileManager(getFileSystemOpts(), VirtualFileSystem);
 }
 
 // Source Manager
@@ -867,6 +875,8 @@ static void compileModule(CompilerInstance &ImportingInstance,
                                    ImportingInstance.getDiagnosticClient()),
                              /*ShouldOwnClient=*/true);
 
+  Instance.setVirtualFileSystem(&ImportingInstance.getVirtualFileSystem());
+
   // Note that this module is part of the module build stack, so that we
   // can detect cycles in the module graph.
   Instance.createFileManager(); // FIXME: Adopt file manager from importer?