swift234

Put alignas(N) on classes with strict alignment.

We use three tag bits on Expr*, Stmt*, Decl*, TypeBase* and SILTypeInfo*, and four on DeclContext*, so set the alignment of the pointed-to types formally with alignas(N) instead of relying on operator new passing down the right alignment to the allocator. Get rid of the informal T::Alignment members of these classes and pass alignof(T) to their allocators. Fix the 'operator new' of DeclContext subclasses so that we can actually use the four tag bits PointerLikeTypeTraits<DeclContext*> claims are available.

Swift SVN r4587