Don't complain about Objective-C method conflicts with unavailable requirements.

Specifically, method requirements that become unnecessary because they
get superseded by the subscript they imply. Fixes
rdar://problem/21420656.

Swift SVN r30194

Author: DougGregor

lib/Sema/TypeCheckProtocol.cpp

@@ -1663,7 +1663,8 @@ void ConformanceChecker::recordOptionalWitness(ValueDecl *requirement) {
 
   // If the requirement is @objc, note that we have an unsatisfied
   // optional @objc requirement.
-  if (requirement->isObjC()) {
+  if (requirement->isObjC() &&
+      !requirement->getAttrs().isUnavailable(TC.Context)) {
     // If we're not suppressing diagnostics, look for a non-@objc near
     // match.
     if (!SuppressDiagnostics)

test/ClangModules/Inputs/custom-modules/ObjCSubscripts.h

@@ -20,3 +20,12 @@
 - (nullable NSString *)objectForKeyedSubscript:(nonnull NSArray *)subscript;
 - (void)setObject:(nullable NSString *)object forKeyedSubscript:(nonnull NSArray *)key;
 @end
+
+@protocol KeySubscriptProto1
+- (nullable NSString *)objectForKeyedSubscript:(nonnull NSString *)subscript;
+@end
+
+@protocol KeySubscriptProto2
+- (NSString *)objectForKeyedSubscript:(NSString *)subscript;
+- (void)setObject:(NSString *)object forKeyedSubscript:(NSString *)key;
+@end

test/ClangModules/objc_subscript.swift

@@ -40,3 +40,16 @@ class KeySubscript4Sub : KeySubscript4 {
     set { }
   }
 }
+
+class ConformsToKeySubscriptProto1 : KeySubscriptProto1 {
+  @objc subscript (s: String) -> String? {
+    return s
+  }
+}
+
+class ConformsToKeySubscriptProto2 : KeySubscriptProto2 {
+  @objc subscript (s: String!) -> String! {
+    get { return s }
+    set { }
+  }
+}