Sema: Make "@retroactive attribute does not apply" a warning.

Follow up to #71302.

Resolves rdar://122590301

Author: tshortli

include/swift/AST/DiagnosticsSema.def

@@ -2544,9 +2544,9 @@ ERROR(retroactive_not_in_extension_inheritance_clause,none,
       "'retroactive' attribute only applies in inheritance clauses in "
       "extensions", ())
 
-ERROR(retroactive_attr_does_not_apply,none,
-      "'retroactive' attribute does not apply; %0 is declared in this module",
-      (Identifier))
+WARNING(retroactive_attr_does_not_apply,none,
+        "'retroactive' attribute does not apply; %0 is declared in this module",
+        (Identifier))
 
 WARNING(extension_retroactive_conformance,none,
        "extension declares a conformance of imported type %0 to imported "

test/Sema/extension_retroactive_conformances.swift

@@ -65,9 +65,9 @@ extension Sample1: ClientProtocol {}
 
 struct Sample7: @retroactive SampleProtocol1 {} // expected-error {{'retroactive' attribute only applies in inheritance clauses in extensions}}
 
-extension Sample7: @retroactive ClientProtocol {} // expected-error {{'retroactive' attribute does not apply; 'Sample7' is declared in this module}}
+extension Sample7: @retroactive ClientProtocol {} // expected-warning {{'retroactive' attribute does not apply; 'Sample7' is declared in this module}}
 
-extension Int: @retroactive ClientProtocol {} // expected-error {{'retroactive' attribute does not apply; 'ClientProtocol' is declared in this module}}
+extension Int: @retroactive ClientProtocol {} // expected-warning {{'retroactive' attribute does not apply; 'ClientProtocol' is declared in this module}}
 
 func f(_ x: @retroactive Int) {} // expected-error {{'retroactive' attribute only applies in inheritance clauses in extensions}}