[Macros] Always parse macro expansions, diagnose later

Always parse macro expansions, regardless of language mode, and
eliminate the fallback path for very, very, very old object literals
like `#Color`. Instead, check for the feature flag for macro
declaration and at macro expansion time, since this is a semantic
restriction.

While here, refactor things so the vast majority of the macro-handling
logic still applies even if the Swift Swift parser is disabled. Only
attempts to expand the macro will fail. This allows us to enable the
macro-diagnostics test everywhere.

lib/Parse/ParseDecl.cpp

@@ -4844,11 +4844,11 @@ Parser::parseDecl(ParseDeclOptions Flags,
       consumeToken(tok::code_complete);
       DeclResult = makeParserCodeCompletionResult<Decl>();
       break;
-    } else if (Context.LangOpts.hasFeature(Feature::Macros)) {
-      DeclResult = parseDeclMacroExpansion(Flags, Attributes);
-      break;
     }
-    LLVM_FALLTHROUGH;
+
+    // Parse as a macro expansion.
+    DeclResult = parseDeclMacroExpansion(Flags, Attributes);
+    break;
 
   case tok::pound_if:
   case tok::pound_sourceLocation: