AST: Don't ignore alignas on EnumDecls when calculating alignment

We didn't consider any alignment attributes on an EnumDecl when
calculating alignment.

While we are here, ignore alignment specifications on typedef types if
one is used as the underlying type.  Otherwise, weird things happen:

enum Y : int;
Y y;

typedef int __attribute__((aligned(64))) u;
enum Y : u {};

What is the alignment of 'Y'?  It would be more consistent with the
overall design of enums with fixed underlying types to consider the
underlying type's UnqualifiedDesugaredType.

This fixes PR22279.

llvm-svn: 226653