[Diagnostics] Tailor using diagnostic to current use-case - default isolation

(cherry picked from commit ec9132c)

Author: xedin

include/swift/AST/DiagnosticsParse.def

@@ -2184,8 +2184,8 @@ ERROR(nonisolated_nonsending_repeated,none,
 // MARK: using @<attribute> or using <identifier>
 //------------------------------------------------------------------------------
 ERROR(using_decl_invalid_specifier,PointsToFirstBadToken,
-      "'using' declaration does not support %0 %select{modifier|attribute}1",
-      (Identifier, bool))
+      "default isolation can only be set to '@MainActor' or 'nonisolated'",
+      ())
 ERROR(experimental_using_decl_disabled,PointsToFirstBadToken,
       "'using' is an experimental feature that is currently disabled", ())
 

lib/Parse/ParseDecl.cpp

@@ -6680,8 +6680,7 @@ ParserResult<UsingDecl> Parser::parseDeclUsing(ParseDeclOptions Flags,
           .Default(std::nullopt);
 
   if (!Specifier) {
-    diagnose(SpecifierLoc, diag::using_decl_invalid_specifier, RawSpecifier,
-             AtLoc.isValid());
+    diagnose(SpecifierLoc, diag::using_decl_invalid_specifier);
     return nullptr;
   }
 

test/Parse/using.swift

@@ -10,8 +10,8 @@ using @MainActor
 using nonisolated
 // expected-error@-1 {{invalid redeclaration of file-level default actor isolation}}
 
-using @Test // expected-error {{'using' declaration does not support 'Test' attribute}}
-using test // expected-error {{'using' declaration does not support 'test' modifier}}
+using @Test // expected-error {{default isolation can only be set to '@MainActor' or 'nonisolated'}}
+using test // expected-error {{default isolation can only be set to '@MainActor' or 'nonisolated'}}
 
 do {
   using // expected-warning {{expression of type 'Int' is unused}}