[BitwiseCopyable] Infer and check constraint.

When the BitwiseCopyable experimental feature is enabled, infer types to
conform to `_BitwiseCopyable`.  The `_BitwiseCopyable` inference broadly
follows the approach taken to infer `Sendable`.

(1) Special types are conformed:
- function types if trivial
- metatypes
- builtin types if trivial

(2) TheTupleType is conditionally conformed.

(3) Nominal types are conformed if:
- non-public or public+fixed-layout
- enum or struct (non-class)
- every field conforms to _BitwiseCopyable

Additionally, check that nominal types which are explicitly conformed to
`_BitwiseCopyable` satisfy the latter two conditions of (3).

For a public, non-fixed-layout type to conform to `_BitwiseCopyable`,
the user must conform the type explicitly.

Finally, verify that conformances correspond to TypeLowering's notion of
triviality to the appropriate extent:
- if a type isn't trivial, it doesn't conform to `_BitwiseCopyable`
  unless it's an archetype
- if a type is trivial, it conforms to `_BitwiseCopyable` unless some
  field in its layout doesn't conform to `_BitwiseCopyable`, which is
  only permitted under certain circumstances (the type has generic
  parameters, the type is public non-fixed-layout, the type is a
  reference but has ReferenceStorage::Unmanaged, the type is a
  ModuleType, etc.)

lib/AST/ProtocolConformance.cpp

@@ -1258,6 +1258,11 @@ static SmallVector<ProtocolConformance *, 2> findSynthesizedConformances(
       trySynthesize(KnownProtocolKind::Copyable);
       trySynthesize(KnownProtocolKind::Escapable);
     }
+
+    if (nominal->getASTContext().LangOpts.hasFeature(
+            Feature::BitwiseCopyable)) {
+      trySynthesize(KnownProtocolKind::BitwiseCopyable);
+    }
   }
 
   /// Distributed actors can synthesize Encodable/Decodable, so look for those