Basic: Return a reference from FileManager::getVirtualFileSystem, NFC

FileManager constructs a VFS in its constructor if it isn't passed one,
and there's no way to reset it.  Make that contract clear by returning a
reference from its accessor.

https://reviews.llvm.org/D59388

llvm-svn: 357038
diff --git a/clang/unittests/AST/ASTImporterTest.cpp b/clang/unittests/AST/ASTImporterTest.cpp
index d6bd134..a282e2a 100644
--- a/clang/unittests/AST/ASTImporterTest.cpp
+++ b/clang/unittests/AST/ASTImporterTest.cpp
@@ -44,7 +44,7 @@
   assert(ToAST);
   ASTContext &ToCtx = ToAST->getASTContext();
   auto *OFS = static_cast<llvm::vfs::OverlayFileSystem *>(
-      ToCtx.getSourceManager().getFileManager().getVirtualFileSystem().get());
+      &ToCtx.getSourceManager().getFileManager().getVirtualFileSystem());
   auto *MFS = static_cast<llvm::vfs::InMemoryFileSystem *>(
       OFS->overlays_begin()->get());
   MFS->addFile(FileName, 0, std::move(Buffer));