Don't complain about near misses for declarations that are overrides.

An overriding declaration doesn't have a choice about its signature,
because the signature is dictated by the overridden
declaration. Therefore, don't produce a warning for it.

Fixes rdar://problem/28524237.

Author: DougGregor

lib/Sema/TypeCheckProtocol.cpp

@@ -4858,7 +4858,11 @@ void TypeChecker::checkConformancesInContext(DeclContext *dc,
       if (!value) continue;
       if (isa<TypeDecl>(value)) continue;
       if (!value->getFullName()) continue;
-      
+
+      // If this declaration overrides another declaration, the signature is
+      // fixed; don't complain about near misses.
+      if (value->getOverriddenDecl()) continue;
+
       // If this member is a witness to any @objc requirement, ignore it.
       if (!findWitnessedObjCRequirements(value, /*anySingleRequirement=*/true)
             .empty())

test/decl/protocol/conforms/near_miss_objc.swift

@@ -119,7 +119,7 @@ class C7a : P7 {
   // expected-note@-4{{make 'method(foo:)' private to silence this warning}}
 }
 
-// Don't complain about near-misses that satisfy other protocol
+// Don't complain about near misses that satisfy other protocol
 // requirements.
 @objc protocol P8 {
   @objc optional func foo(exactMatch: Int)
@@ -134,3 +134,13 @@ class C8Super : P8 { }
 class C9Sub : C8Super, P9 {
   func foo(exactMatch: Int) { }
 }
+
+// Don't complain about overriding methods that are near misses;
+// the user cannot make it satisfy the protocol requirement.
+class C10Super {
+  func foo(nearMatch: Int) { }
+}
+
+class C10Sub : C10Super, P8 {
+  override func foo(nearMatch: Int) { }
+}