Sema: Tweak a diagnostic

We talk about 'protocol refinement' rather than 'protocol inheritance';
but in this case, a more specific phrase makes even more sense, I think.

Author: slavapestov

include/swift/AST/DiagnosticsSema.def

@@ -2628,7 +2628,7 @@ WARNING(duplicate_anyobject_class_inheritance,none,
 ERROR(inheritance_from_protocol_with_superclass,none,
   "inheritance from class-constrained protocol composition type %0", (Type))
 WARNING(anyobject_class_inheritance_deprecated,none,
-  "using 'class' keyword for protocol inheritance is deprecated; "
+  "using 'class' keyword to define a class-constrained protocol is deprecated; "
   "use 'AnyObject' instead", ())
 ERROR(multiple_inheritance,none,
   "multiple inheritance from classes %0 and %1", (Type, Type))

test/decl/inherit/inherit_anyobject_protocol_swift5.swift

@@ -1,10 +1,10 @@
 // RUN: %target-typecheck-verify-swift -swift-version 5
 
 protocol P: class {}
-// expected-warning@-1 {{using 'class' keyword for protocol inheritance is deprecated; use 'AnyObject' instead}} {{13-18=AnyObject}} 
+// expected-warning@-1 {{using 'class' keyword to define a class-constrained protocol is deprecated; use 'AnyObject' instead}} {{13-18=AnyObject}}
 protocol P1: AnyObject {}
 protocol P2 {}
 protocol P3: class, P2 {} 
-// expected-warning@-1 {{using 'class' keyword for protocol inheritance is deprecated; use 'AnyObject' instead}} {{14-19=AnyObject}}
+// expected-warning@-1 {{using 'class' keyword to define a class-constrained protocol is deprecated; use 'AnyObject' instead}} {{14-19=AnyObject}}
 protocol P4: P2, class {} // expected-error {{'class' must come first in the requirement list}}
-// expected-warning@-1 {{using 'class' keyword for protocol inheritance is deprecated; use 'AnyObject' instead}} {{18-23=AnyObject}}
+// expected-warning@-1 {{using 'class' keyword to define a class-constrained protocol is deprecated; use 'AnyObject' instead}} {{18-23=AnyObject}}