[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.

include/swift/AST/DiagnosticsCommon.def

@@ -219,6 +219,8 @@ ERROR(unknown_attribute,none,
 //------------------------------------------------------------------------------
 NOTE(in_macro_expansion,none,
      "in expansion of macro %0 here", (DeclName))
+ERROR(macro_experimental,none,
+      "macros are an experimental feature that is not enabled", ())
 
 //------------------------------------------------------------------------------
 // MARK: bridged diagnostics

include/swift/AST/DiagnosticsParse.def

@@ -1329,9 +1329,6 @@ ERROR(expected_rsquare_array_expr,PointsToFirstBadToken,
 // Object literal expressions
 ERROR(expected_arg_list_in_object_literal,PointsToFirstBadToken,
       "expected argument list in object literal", ())
-ERROR(legacy_object_literal,none,
-      "'%select{|[}0#%1(...)%select{|#]}0' has been renamed to '#%2(...)'",
-      (bool, StringRef, StringRef))
 
 // Unknown pound expression.
 ERROR(unknown_pound_expr,none,

include/swift/AST/DiagnosticsSema.def

@@ -6762,8 +6762,7 @@ ERROR(experimental_no_metadata_feature_can_only_be_used_when_enabled,
 ERROR(expected_macro_expansion_expr,PointsToFirstBadToken,
       "expected macro expansion to produce an expression", ())
 ERROR(macro_undefined,PointsToFirstBadToken,
-      "macro %0 is undefined; use `-load-plugin-library` to specify dynamic "
-      "libraries that contain this macro", (Identifier))
+      "no macro named %0", (Identifier))
 ERROR(external_macro_not_found,none,
       "external macro implementation type '%0.%1' could not be found for "
       "macro %2; the type must be public and provided via "
@@ -6798,6 +6797,8 @@ ERROR(macro_expansion_missing_pound,none,
       "expansion of macro %0 requires leading '#'", (DeclName))
 ERROR(macro_expansion_missing_arguments,none,
       "expansion of macro %0 requires arguments", (DeclName))
+ERROR(macro_unsupported,none,
+      "macros are not supported in this compiler", ())
 
 //------------------------------------------------------------------------------
 // MARK: Move Only Errors

include/swift/Parse/Parser.h

@@ -1748,7 +1748,6 @@ public:
   ParserResult<Expr> parseExprMacroExpansion(bool isExprBasic);
   ParserResult<Expr> parseExprCollection();
   ParserResult<Expr> parseExprCollectionElement(Optional<bool> &isDictionary);
-  ParserResult<Expr> parseExprPoundUnknown(SourceLoc LSquareLoc);
   ParserResult<Expr>
   parseExprPoundCodeCompletion(Optional<StmtKind> ParentKind);