Fix SR-718: Type mismatch reported as extraneous parameter

The issue here is that the constraint solver was deciding on
FixKind::RelabelCallTuple as the fix for the problem and emitting the
diagnostic, even though there were two different fixes possible.

CSDiags has the infrastructure to support doing doing the right thing
here, but is only being used for ApplyExprs, not SubscriptExprs.

The solution is to fix both problems: remove FixKind::RelabelCallTuple,
to let CSDiags handle the problem, and enhance CSDiags to treat
SubscriptExpr more commonly with ApplyExpr.  This improves several cases
where the solver was picking one solution randomly and suggesting that
as a fix, instead of listing that there are multiple different solutions.

Author: lattner

lib/Sema/CSApply.cpp

@@ -6182,7 +6182,6 @@ bool ConstraintSystem::applySolutionFix(Expr *expr,
 
   case FixKind::TupleToScalar: 
   case FixKind::ScalarToTuple:
-  case FixKind::RelabelCallTuple:
     return diagnoseArgumentLabelError(affected,
                                       fix.first.getRelabelTupleNames(*this),
                            /*isSubscript=*/locator->getPath().back().getKind()