[BitwiseCopyable] Loosen validation assertion.

A function can return `some BitwiseCopyable`.  The resulting value is
(currently) non-trivial but conforms to `BitwiseCopyable`.

rdar://125443922

Author: nate-chandler

lib/SIL/IR/TypeLowering.cpp

@@ -3205,9 +3205,10 @@ void TypeConverter::verifyTrivialLowering(const TypeLowering &lowering,
 
   if (!lowering.isTrivial() && conformance) {
     // A non-trivial type can have a conformance in a few cases:
-    // (1) contains a conforming archetype
+    // (1) containing or being a conforming archetype
     // (2) is resilient with minimal expansion
     // (3) containing or being ~Escapable
+    // (4) containing or being an opaque archetype
     bool hasNoConformingArchetypeNode = visitAggregateLeaves(
         origType, substType, forExpansion,
         /*isLeaf=*/
@@ -3256,6 +3257,11 @@ void TypeConverter::verifyTrivialLowering(const TypeLowering &lowering,
           if (origTy.isTypeParameter())
             return false;
 
+          // An opaque archetype may conform but be non-trivial (case (4)).
+          if (isa<OpaqueTypeArchetypeType>(ty)) {
+            return false;
+          }
+
           return true;
         });
     if (hasNoConformingArchetypeNode) {

test/SILGen/bitwise_copyable.swift

@@ -47,3 +47,7 @@ public enum E : _BitwiseCopyable {
 func take<T : _BitwiseCopyable>(_ t: T) {}
 
 func pass(_ e: E) { take(e) }
+
+func opacify() -> some _BitwiseCopyable {
+    return Int()
+}