NCGenerics: narrow use of custom diagnostics

In some cases we get a `ConstraintKind::SelfObjectOfProtocol` here.
That's typically for an existential cast. In such a case, we actually
get a better standard diagnostic than what MustBeCopyable can produce
currently. So this change narrows things to what MustBeCopyable is
better at diagnosing.

Author: kavon

lib/Sema/CSSimplify.cpp

@@ -8705,7 +8705,8 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyConformsToConstraint(
     }
 
     // If this is a failure to conform to Copyable, tailor the error message.
-    if (protocol->isSpecificProtocol(KnownProtocolKind::Copyable)) {
+    if (kind == ConstraintKind::ConformsTo &&
+        protocol->isSpecificProtocol(KnownProtocolKind::Copyable)) {
       auto *fix =
           MustBeCopyable::create(*this,
                                  type,