Allow '@objc(RuntimeName)' on classes with generic ancestry.

This is accomplished by recognizing this specific situation and
replacing the 'objc' attribute with a hidden '_objcRuntimeName'
attribute. This /only/ applies to classes that are themselves
non-generic (including any enclosing generic context) but that have
generic ancestry, and thus cannot be exposed directly to Objective-C.

This commit also eliminates '@NSKeyedArchiverClassName'. It was
decided that the distinction between '@NSKeyedArchiverClassName' and
'@objc' was too subtle to be worth explaining to developers, and that
any case where you'd use '@NSKeyedArchiverClassName' was already a
place where the ObjC name wasn't visible at compile time.

This commit does not update diagnostics to reflect this change; we're
going to change them anyway.

rdar://problem/32414557

include/swift/Serialization/ModuleFormat.h

@@ -1321,8 +1321,8 @@ namespace decls_block {
   using SynthesizedProtocolDeclAttrLayout
     = BCRecordLayout<SynthesizedProtocol_DECL_ATTR>;
   using ImplementsDeclAttrLayout = BCRecordLayout<Implements_DECL_ATTR>;
-  using NSKeyedArchiverClassNameDeclAttrLayout
-    = BCRecordLayout<NSKeyedArchiverClassName_DECL_ATTR>;
+  using ObjCRuntimeNameDeclAttrLayout
+    = BCRecordLayout<ObjCRuntimeName_DECL_ATTR>;
 
   using InlineDeclAttrLayout = BCRecordLayout<
     Inline_DECL_ATTR,