Sema: Fix incorrect usage of NormalProtocolConformance in Codable synthesis

Noticed by inspection. It looks like there are no tests for generic
Codable types. I'm just adding one to cover the changed code path.

Author: slavapestov

lib/Sema/TypeCheckDecl.cpp

@@ -8822,8 +8822,7 @@ void TypeChecker::synthesizeMemberForLookup(NominalTypeDecl *target,
     if (auto ref = conformsToProtocol(targetType, protocol, target,
                                       ConformanceCheckFlags::Used,
                                       SourceLoc())) {
-      if (auto *conformance =
-          dyn_cast_or_null<NormalProtocolConformance>(ref->getConcrete())) {
+      if (auto *conformance = ref->getConcrete()->getRootNormalConformance()) {
         if (conformance->isIncomplete()) {
           // Check conformance, forcing synthesis.
           //

test/decl/protocol/special/coding/struct_codable_member_type_lookup.swift

@@ -645,3 +645,7 @@ extension C.Inner {
     case value
   }
 }
+
+struct GenericCodableStruct<T : Codable> : Codable {}
+
+func foo(_: GenericCodableStruct<Int>.CodingKeys) // expected-error {{'CodingKeys' is inaccessible due to 'private' protection level}}