swift234

Fold the rest of PointerLikeTypeTraitsFwdDecl.h into TypeAlignments.h.

TypeAlignments.h predates this whole mess; it was used for types with
stronger alignment in PointerLikeTypeTraits than the old default of
"2 by fiat and assumption". All remaining forward-declared types are
AST types, so fold them into TypeAlignments.h.

(The one exception is SILTypeList.h, but that's already gone on master.)

To avoid future ODR issues, explicitly include TypeAlignments.h into
every header that defines a type it forward-declares.

I wish we could use partial specialization to provide PointerLikeTypeTraits
for all derived classes of Decl, TypeBase, etc, but that's not something
you can do in C++ if you don't control the traits class.