[RequirementMachine] Only emit a redundancy warning for layout constraints

if the type is constrained to a class layout, and add a few more test cases
for type requirement conflicts.

Author: hborla

lib/AST/RequirementMachine/RequirementLowering.cpp

@@ -152,7 +152,7 @@ static void desugarLayoutRequirement(Type subjectType,
   if (!subjectType->isTypeParameter()) {
     Requirement requirement(RequirementKind::Layout,
                             subjectType, layout);
-    if (subjectType->isAnyClassReferenceType()) {
+    if (layout->isClass() && subjectType->isAnyClassReferenceType()) {
       errors.push_back(
           RequirementError::forRedundantRequirement(requirement, loc));
     } else {

test/Generics/requirement_machine_diagnostics.swift

@@ -59,10 +59,17 @@ class C {}
 func concreteLayoutRedundancy<T>(_: T) where C : AnyObject {}
 // expected-warning@-1{{redundant constraint 'C' : 'AnyObject'}}
 
+func concreteLayoutConflict<T>(_: T) where Int : AnyObject {}
+// expected-error@-1{{type 'Int' in conformance requirement does not refer to a generic parameter or associated type}}
+
 class C2: C {}
 func concreteSubclassRedundancy<T>(_: T) where C2 : C {}
 // expected-warning@-1{{redundant superclass constraint 'C2' : 'C'}}
 
+class D {}
+func concreteSubclassConflict<T>(_: T) where D : C {}
+// expected-error@-1{{type 'D' in conformance requirement does not refer to a generic parameter or associated type}}
+
 protocol UselessProtocolWhereClause where Int == Int {}
 // expected-warning@-1 {{redundant same-type constraint 'Int' == 'Int'}}