Treat alignas and _Alignas as keyword attributes. This allows us to
pretty-print them properly (modulo the more general badness in alignment
attribute printing).
<rdar://problem/12552374>

Replacing the address argument type with address-expression in cases where StringToAddress() is used, and hence an expression can be passed where previously only a numeric address was allowed
This makes the documentation more clear and helps users discover that they can truly pass in an expression in these situations.