[Omit needless words] Don't ask for the StringRef of an empty identifier.

Author: DougGregor

lib/Sema/MiscDiagnostics.cpp

@@ -2068,7 +2068,7 @@ static Optional<DeclName> omitNeedlessWords(AbstractFunctionDecl *afd) {
   // Figure out the first parameter name.
   StringRef firstParamName;
   auto params = afd->getParameterList(afd->getImplicitSelfDecl() ? 1 : 0);
-  if (params->size() != 0)
+  if (params->size() != 0 && !params->get(0)->getName().empty())
     firstParamName = params->get(0)->getName().str();
 
   // Find the set of property names.

test/Sema/omit_needless_words.swift

@@ -30,3 +30,5 @@ extension String {
 class NSArray {
   func arrayByAddingObject(x: AnyObject) -> NSArray { return NSArray() } // expected-warning{{'arrayByAddingObject' could be named 'adding' [-Womit-needless-words]}}{{8-27=adding}}
 }
+
+func emptyFirstParamName(_: Int) { }