[ConstraintSystem] Downgrade some invalid specialization uses to a warning until Swift 6 language mode

Some invalid specializations were previously allowed by the compiler
and we found some existing code that used that (albeit invalid) syntax,
so we need to stage that error as a warning until Swift 6 language mode
to avoid source compatibility break.

Resolves: rdar://134740240

lib/Sema/CSSimplify.cpp

@@ -14069,6 +14069,13 @@ ConstraintSystem::simplifyExplicitGenericArgumentsConstraint(
 
   auto genericParams = getGenericParams(decl);
   if (!decl->getAsGenericContext() || !genericParams) {
+    if (isa<AbstractFunctionDecl>(decl)) {
+      return recordFix(AllowFunctionSpecialization::create(
+                 *this, decl, getConstraintLocator(locator)))
+                 ? SolutionKind::Error
+                 : SolutionKind::Solved;
+    }
+
     // Allow concrete macros to have specializations with just a warning.
     return recordFix(AllowConcreteTypeSpecialization::create(
                *this, type1, decl, getConstraintLocator(locator),
@@ -14111,10 +14118,7 @@ ConstraintSystem::simplifyExplicitGenericArgumentsConstraint(
   // FIXME: We could support explicit function specialization.
   if (openedGenericParams.empty() ||
       (isa<AbstractFunctionDecl>(decl) && !hasParameterPack)) {
-    if (!shouldAttemptFixes())
-      return SolutionKind::Error;
-
-    return recordFix(AllowGenericFunctionSpecialization::create(
+    return recordFix(AllowFunctionSpecialization::create(
                *this, decl, getConstraintLocator(locator)))
                ? SolutionKind::Error
                : SolutionKind::Solved;
@@ -15246,7 +15250,7 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyFixConstraint(
   case FixKind::AllowAssociatedValueMismatch:
   case FixKind::GenericArgumentsMismatch:
   case FixKind::AllowConcreteTypeSpecialization:
-  case FixKind::AllowGenericFunctionSpecialization:
+  case FixKind::AllowFunctionSpecialization:
   case FixKind::IgnoreGenericSpecializationArityMismatch:
   case FixKind::IgnoreKeyPathSubscriptIndexMismatch:
   case FixKind::AllowMemberRefOnExistential: {