[Constraint solver] Slightly expand the "contextual conversion failure" check.

Expand the context conversion failure check used to fix
rdar://28909024 to cover the case where we have a single-argument
mismatch and there is a type parameter deduction as well.

Author: DougGregor

lib/Sema/CSDiag.cpp

@@ -5436,7 +5436,9 @@ bool FailureDiagnosis::visitApplyExpr(ApplyExpr *callExpr) {
     if (calleeInfo.closeness == CC_ExactMatch)
       return true;
 
-    if (!CS->getContextualType() || calleeInfo.closeness != CC_ArgumentMismatch)
+    if (!CS->getContextualType() ||
+        (calleeInfo.closeness != CC_ArgumentMismatch &&
+         calleeInfo.closeness != CC_OneGenericArgumentMismatch))
       return false;
 
     CalleeCandidateInfo candidates(fnExpr, hasTrailingClosure, CS);