[Diagnostics] Improve simplifyLocator to extract arguments from unresolved member calls

Unresolved member calls e.g. `.foo(1, 2)` are unique in a way that
they don't form regular application expressions. So let's teach
`simplifyLocator` how to extract arguments from such calls
based on locators like:
`[UnresolvedMemberExpr -> apply argument -> comparing call argument # to parameter #]`.

This helps to diagnose more failures via diagnostics attached to
constraint system fixes.

Author: xedin

lib/Sema/CSDiag.cpp

@@ -147,7 +147,7 @@ void constraints::simplifyLocator(Expr *&anchor,
 
   while (!path.empty()) {
     switch (path[0].getKind()) {
-    case ConstraintLocator::ApplyArgument:
+    case ConstraintLocator::ApplyArgument: {
       // Extract application argument.
       if (auto applyExpr = dyn_cast<ApplyExpr>(anchor)) {
         // The target anchor is the function being called.
@@ -167,7 +167,20 @@ void constraints::simplifyLocator(Expr *&anchor,
         path = path.slice(1);
         continue;
       }
+
+      if (auto *UME = dyn_cast<UnresolvedMemberExpr>(anchor)) {
+        // The target anchor is the method being called,
+        // no additional information could be retrieved
+        // about this call.
+        targetAnchor = nullptr;
+        targetPath.clear();
+
+        anchor = UME->getArgument();
+        path = path.slice(1);
+        continue;
+      }
       break;
+    }
 
     case ConstraintLocator::ApplyFunction:
       // Extract application function.
@@ -317,7 +330,7 @@ void constraints::simplifyLocator(Expr *&anchor,
       // This was just for identifying purposes, strip it off.
       path = path.slice(1);
       continue;
-        
+
     default:
       // FIXME: Lots of other cases to handle.
       break;

test/Constraints/diagnostics.swift

@@ -447,7 +447,7 @@ let _: Color = .overload(1)  // expected-error {{ambiguous reference to member '
 let _: Color = .frob(1.0, &i) // expected-error {{missing argument label 'b:' in call}}
 let _: Color = .frob(1.0, b: &i) // expected-error {{cannot convert value of type 'Double' to expected argument type 'Int'}}
 let _: Color = .frob(1, i)  // expected-error {{missing argument label 'b:' in call}}
-let _: Color = .frob(1, b: i)  // expected-error {{passing value of type 'Int' to an inout parameter requires explicit '&'}}
+let _: Color = .frob(1, b: i)  // expected-error {{passing value of type 'Int' to an inout parameter requires explicit '&'}} {{28-28=&}}
 let _: Color = .frob(1, &d) // expected-error {{missing argument label 'b:' in call}}
 let _: Color = .frob(1, b: &d) // expected-error {{cannot convert value of type 'Double' to expected argument type 'Int'}}
 var someColor : Color = .red // expected-error {{enum type 'Color' has no case 'red'; did you mean 'Red'}}