[Demangler] Eliminate a use-of-uninitialized in misuse of StringRef.

Putting the results of StringRef::data() back into a StringRef scans for the
null at the end, which won’t always be there. Caught by ASan.

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);