swift234

AST: Add AbstractStorageDecl::hasFixedLayout(), NFC

This is a preliminary cleanup before adding resilient access to
global variables. It might also enable @_fixed_layout properties
on resilient structs one day, if we choose to do that.

Also, change NominalTypeDecl::hasFixedLayout() to not care about
classes imported from Clang. IRGen already has its own fine-grained
queries and abstractions for asking this question, so don't try
to capture in the AST.