[Conformance] Always downgrade redundant conformances to marker protocols to

a warning.

Previous compiler versions allowed this, so we should stage the change in as
a warning. This was already a warning across modules, so this change only impacts
redundant conformances to marker protocols within a module. This code also isn't
particularly harmful, because marker protocols don't have requirements, so there
isn't the same risk of unexpected behavior as other redundant conformances.

(cherry picked from commit 5eb16ad)

Author: hborla

lib/Sema/TypeCheckProtocol.cpp

@@ -6283,7 +6283,7 @@ void TypeChecker::checkConformancesInContext(IterableDeclContext *idc) {
     }
 
     if ((existingModule != dc->getParentModule() && conformanceInOrigModule) ||
-        isSendable) {
+        diag.Protocol->isMarkerProtocol()) {
       // Warn about the conformance.
       if (isSendable && SendableConformance &&
           isa<InheritedProtocolConformance>(SendableConformance)) {

test/decl/protocol/conforms/redundant_conformance.swift

@@ -86,3 +86,10 @@ class Class3 {
 class SomeMockClass: Class3.ProviderThree { // okay
   var someInt: Int = 5
 }
+
+
+class ImplicitCopyable {}
+
+class InheritImplicitCopyable: ImplicitCopyable, Copyable {}
+// expected-warning@-1 {{redundant conformance of 'InheritImplicitCopyable' to protocol 'Copyable'}}
+// expected-note@-2 {{'InheritImplicitCopyable' inherits conformance to protocol 'Copyable' from superclass here}}