[ConstraintSystem] Change missingArgument callback to produce an index of synthesized argument

In diagnostic mode allow argument matcher to synthesize new
arguments, which makes it much easier to diagnose problems
related to missing arguments.

Author: xedin

lib/Sema/CSDiag.cpp

@@ -2211,7 +2211,7 @@ class ArgumentMatcher : public MatchCallArgumentListener {
     Diagnosed = true;
   }
 
-  void missingArgument(unsigned missingParamIdx) override {
+  Optional<unsigned> missingArgument(unsigned missingParamIdx) override {
     auto &param = Parameters[missingParamIdx];
     Identifier name = param.getLabel();
 
@@ -2341,6 +2341,7 @@ class ArgumentMatcher : public MatchCallArgumentListener {
                   candidate.getDecl()->getFullName());
 
     Diagnosed = true;
+    return None;
   }
 
   bool isPropertyWrapperImplicitInit() {

lib/Sema/CSSimplify.cpp

@@ -36,7 +36,10 @@ MatchCallArgumentListener::~MatchCallArgumentListener() { }
 
 void MatchCallArgumentListener::extraArgument(unsigned argIdx) { }
 
-void MatchCallArgumentListener::missingArgument(unsigned paramIdx) { }
+Optional<unsigned>
+MatchCallArgumentListener::missingArgument(unsigned paramIdx) {
+  return None;
+}
 
 bool MatchCallArgumentListener::missingLabel(unsigned paramIdx) { return true; }
 bool MatchCallArgumentListener::extraneousLabel(unsigned paramIdx) {

lib/Sema/CalleeCandidateInfo.cpp

@@ -284,8 +284,9 @@ CalleeCandidateInfo::ClosenessResultTy CalleeCandidateInfo::evaluateCloseness(
     void extraArgument(unsigned argIdx) override {
       result = CC_ArgumentCountMismatch;
     }
-    void missingArgument(unsigned paramIdx) override {
+    Optional<unsigned> missingArgument(unsigned paramIdx) override {
       result = CC_ArgumentCountMismatch;
+      return None;
     }
     bool missingLabel(unsigned paramIdx) override {
       result = CC_ArgumentLabelMismatch;

lib/Sema/ConstraintSystem.h

@@ -3858,7 +3858,7 @@ class MatchCallArgumentListener {
   /// indices.
   ///
   /// \param paramIdx The index of the parameter that is missing an argument.
-  virtual void missingArgument(unsigned paramIdx);
+  virtual Optional<unsigned> missingArgument(unsigned paramIdx);
 
   /// Indicate that there was no label given when one was expected by parameter.
   ///