[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.

Author: hborla

lib/Sema/TypeCheckProtocol.cpp

@@ -6276,7 +6276,7 @@ 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}}