[ConstraintSystem] Remove UR_LabelMismatch overload unavailability reason

This helps with:

- Diagnostics because solver would get more choices to work with
  in diagnostic mode;
- Avoid adding the same overload multiple times
  (retry after label mismatch and no viable candidates);
- Unify overload handling/filtering in `simplfyAppliedOverloads`.

Author: xedin

lib/Sema/CSDiag.cpp

@@ -741,7 +741,6 @@ void FailureDiagnosis::diagnoseUnviableLookupResults(
       return;
 
     switch (firstProblem) {
-    case MemberLookupResult::UR_LabelMismatch:
     case MemberLookupResult::UR_WritableKeyPathOnReadOnlyMember:
     case MemberLookupResult::UR_ReferenceWritableKeyPathOnMutatingMember:
     case MemberLookupResult::UR_KeyPathWithAnyObjectRootType:

lib/Sema/CSSimplify.cpp

@@ -4173,24 +4173,6 @@ performMemberLookup(ConstraintKind constraintKind, DeclName memberName,
   if (!instanceTy->mayHaveMembers())
     return result;
 
-  // If we have a simple name, determine whether there are argument
-  // labels we can use to restrict the set of lookup results.
-  Optional<ArgumentLabelState> argumentLabels;
-  if (memberName.isSimpleName()) {
-    argumentLabels = getArgumentLabels(*this,
-                                       ConstraintLocatorBuilder(memberLocator));
-
-    // If we're referencing AnyObject and we have argument labels, put
-    // the argument labels into the name: we don't want to look for
-    // anything else, because the cost of the general search is so
-    // high.
-    if (baseObjTy->isAnyObject() && argumentLabels) {
-      memberName = DeclName(TC.Context, memberName.getBaseName(),
-                            argumentLabels->Labels);
-      argumentLabels.reset();
-    }
-  }
-
   // Look for members within the base.
   LookupResult &lookup = lookupMember(instanceTy, memberName);
 
@@ -4226,7 +4208,6 @@ performMemberLookup(ConstraintKind constraintKind, DeclName memberName,
       bridgedType = classType;
     }
   }
-  bool labelMismatch = false;
 
   // Local function that adds the given declaration if it is a
   // reasonable choice.
@@ -4291,20 +4272,6 @@ performMemberLookup(ConstraintKind constraintKind, DeclName memberName,
       hasInstanceMethods = true;
     }
 
-    // If the argument labels for this result are incompatible with
-    // the call site, skip it.
-    // FIXME: The subscript check here forces the use of the
-    // function-application simplification logic to handle labels.
-    if (argumentLabels &&
-        (!candidate.isDecl() || !isa<SubscriptDecl>(candidate.getDecl())) &&
-        !areConservativelyCompatibleArgumentLabels(
-            candidate, argumentLabels->Labels,
-            argumentLabels->HasTrailingClosure)) {
-      labelMismatch = true;
-      result.addUnviable(candidate, MemberLookupResult::UR_LabelMismatch);
-      return;
-    }
-
     // If our base is an existential type, we can't make use of any
     // member whose signature involves associated types.
     if (instanceTy->isExistentialType()) {
@@ -4494,8 +4461,6 @@ performMemberLookup(ConstraintKind constraintKind, DeclName memberName,
   };
 
   // Add all results from this lookup.
-retry_after_fail:
-  labelMismatch = false;
   for (auto result : lookup)
     addChoice(getOverloadChoice(result.getValueDecl(),
                                 /*isBridged=*/false,
@@ -4603,14 +4568,6 @@ performMemberLookup(ConstraintKind constraintKind, DeclName memberName,
     }
   }
 
-  // If we rejected some possibilities due to an argument-label
-  // mismatch and ended up with nothing, try again ignoring the
-  // labels. This allows us to perform typo correction on the labels.
-  if (result.ViableCandidates.empty() && labelMismatch && shouldAttemptFixes()){
-    argumentLabels.reset();
-    goto retry_after_fail;
-  }
-
   // If we have no viable or unviable candidates, and we're generating,
   // diagnostics, rerun the query with inaccessible members included, so we can
   // include them in the unviable candidates list.
@@ -4855,7 +4812,6 @@ fixMemberRef(ConstraintSystem &cs, Type baseTy,
                  : nullptr;
     }
 
-    case MemberLookupResult::UR_LabelMismatch:
     // TODO(diagnostics): Add a new fix that is suggests to
     // add `subscript(dynamicMember: {Writable}KeyPath<T, U>)`
     // overload here, that would help if such subscript has

lib/Sema/ConstraintSystem.h

@@ -886,9 +886,6 @@ struct MemberLookupResult {
 
   /// This enum tracks reasons why a candidate is not viable.
   enum UnviableReason {
-    /// Argument labels don't match.
-    UR_LabelMismatch,
-
     /// This uses a type like Self in its signature that cannot be used on an
     /// existential box.
     UR_UnavailableInExistential,