set the alignment of mlir::AttributeStorage to 64 bit explicitly to fix 32 bit platform

On some platform (like WebAssembly), alignof(mlir::AttributeStorage) is 4 instead of 8. As a result, it makes the program crashes since PointerLikeTypeTraits<mlir::Attribute>::NumLowBitsAvailable is 3.

So I explicitly set the alignment of mlir::AttributeStoarge to 64 bits, and set PointerLikeTypeTraits<mlir::Attribute>::NumLowBitsAvailable according to it.

I also fixed an another related error (alignof(NamedAttribute) -> alignof(DictionaryAttributeStorage)) based on reviewer's comments.

Reviewed By: dblaikie, rriddle

Differential Revision: https://reviews.llvm.org/D91062