swift234

[stdlib] Apply @_fixed_layout to various stdlib and overlay types

This is a staging attribute that will eventually mean "fixed-contents"
for structs and "closed" for enums, as described in
docs/LibraryEvolution.rst.

This is pretty much the minimal set of types that must be fixed-layout,
because SILGen makes assumptions about their lowering.

If desired, some SILGen refactoring can allow some of these to be
resilient. For example, bridging value types could be made to work
with resilient types.