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/lib/Lex/PPLexerChange.cpp b/clang/lib/Lex/PPLexerChange.cpp
index 2f1e67c..5f547d9 100644
--- a/clang/lib/Lex/PPLexerChange.cpp
+++ b/clang/lib/Lex/PPLexerChange.cpp
@@ -270,7 +270,7 @@
 
   ModuleMap &ModMap = getHeaderSearchInfo().getModuleMap();
   const DirectoryEntry *Dir = Mod.getUmbrellaDir().Entry;
-  llvm::vfs::FileSystem &FS = *FileMgr.getVirtualFileSystem();
+  llvm::vfs::FileSystem &FS = FileMgr.getVirtualFileSystem();
   std::error_code EC;
   for (llvm::vfs::recursive_directory_iterator Entry(FS, Dir->getName(), EC),
        End;