Define @_implicitly_synthesizes_nested_requirement

State the previously unstated nested type requirement that CodingKeys adds to the witness requirements of a given type. The goal is to make this member cheap to synthesize, and independent of the expensive protocol conformance checks required to append it to the member list.

Further, this makes a clean conceptual separation between what I'm calling "nested type requirements" and actual type and value requirements.

With luck, we'll never have to use this attribute anywhere else.

tools/SourceKit/lib/SwiftLang/SwiftLangSupport.cpp

@@ -785,6 +785,7 @@ Optional<UIdent> SwiftLangSupport::getUIDForDeclAttribute(const swift::DeclAttri
     }
 
     // Ignore these.
+    case DAK_ImplicitlySynthesizesNestedRequirement:
     case DAK_ShowInInterface:
     case DAK_RawDocComment:
     case DAK_HasInitialValue: