Sema: Fix rendering of accessor names in an availability diagnostic.

include/swift/AST/DiagnosticsSema.def

@@ -3520,8 +3520,8 @@ NOTE(multiple_override_prev,none,
      "%0 previously overridden here", (DeclName))
 
 ERROR(override_unavailable, none,
-      "cannot override %0 which has been marked unavailable%select{|: %1}1",
-      (DeclBaseName, StringRef))
+      "cannot override %base0 which has been marked unavailable%select{|: %1}1",
+      (ValueDecl *, StringRef))
 NOTE(suggest_removing_override, none,
      "remove 'override' modifier to declare a new %0",
      (DeclBaseName))

lib/Sema/TypeCheckAvailability.cpp

@@ -1631,7 +1631,7 @@ void swift::diagnoseOverrideOfUnavailableDecl(ValueDecl *override,
   if (attr.getRename().empty()) {
     EncodedDiagnosticMessage EncodedMessage(attr.getMessage());
     diags.diagnose(override, diag::override_unavailable,
-                   override->getBaseName(), EncodedMessage.Message);
+                   override, EncodedMessage.Message);
 
     diags.diagnose(base, diag::availability_marked_unavailable, base);
     return;

test/Availability/availability_unavailable_overrides.swift

@@ -56,8 +56,7 @@ func testAvailableOverrideOfUnavailableDecl() {
 
     override var computedPropertyWithUnavailableSet: Int {
       get { 0 }
-      // FIXME: Diagnostic should refer to "setter for 'computedPropertyWithUnavailableSet'" rather than '_'.
-      set {} // expected-error {{cannot override '_' which has been marked unavailable}}
+      set {} // expected-error {{cannot override setter for 'computedPropertyWithUnavailableSet' which has been marked unavailable}}
     }
   }
 

test/Availability/availability_versions.swift

@@ -917,7 +917,6 @@ class SubWithLimitedMemberAvailability : SuperWithAlwaysAvailableMembers {
     get { return 9 }
     @available(OSX, introduced: 51)
     set(newVal) {} // expected-error {{overriding setter for 'setterShouldAlwaysBeAvailableProperty' must be as available as declaration it overrides}}
-    // This is a terrible diagnostic. rdar://problem/20427938
   }
 
   override var getterShouldAlwaysBeAvailableProperty: Int {