Merge pull request swiftlang#12083 from DougGregor/demangler-past-the-end-read

[Demangler] Eliminate a read-past-the-end-of-a-buffer misuse of StringRef.

Author: DougGregor

lib/Demangling/Context.cpp

@@ -36,7 +36,7 @@ void Context::clear() {
 }
 
 NodePointer Context::demangleSymbolAsNode(llvm::StringRef MangledName) {
-  if (isMangledName(MangledName.data())) {
+  if (isMangledName(MangledName)) {
     return D->demangleSymbol(MangledName);
   }
   return demangleOldSymbolAsNode(MangledName, *D);