Fix a static exclusive diagnostic

Author: Azoy

include/swift/AST/DiagnosticsSema.def

@@ -1929,7 +1929,7 @@ NOTE(attr_static_exclusive_only_type_nonmutating,none,
 ERROR(attr_static_exclusive_only_let_only_param,none,
       "parameter of type %0 must be declared as either 'borrowing' or 'consuming'", (Type))
 ERROR(attr_static_exclusive_only_mutating,none,
-      "@_staticExclusiveOnly type %0 cannot have mutating function %1", (Type, ValueDecl *))
+      "type %0 cannot have mutating function %1", (Type, ValueDecl *))
 
 ERROR(c_func_variadic, none,
       "cannot declare variadic argument %0 in %kind1",

test/attr/attr_static_exclusive_only.swift

@@ -9,7 +9,7 @@ struct B: ~Copyable { // expected-note {{'B' is a non-mutable type}}
                       // expected-note@-2 {{'B' is a non-mutable type}}
                       // expected-note@-3 {{'B' is a non-mutable type}}
                       // expected-note@-4 {{'B' is a non-mutable type}}
-  mutating func change() { // expected-error {{@_staticExclusiveOnly type 'B' cannot have mutating function 'change()'}}
+  mutating func change() { // expected-error {{type 'B' cannot have mutating function 'change()'}}
     print("123")
   }
 }