[NFC] Refactor TableGen for attributes

Summary:
this patch makes tablegen generate llvm attributes in a more generic and simpler (at least to me).

changes:  make tablegen generate
...
ATTRIBUTE_ENUM(Alignment,align)
ATTRIBUTE_ENUM(AllocSize,allocsize)
...
which can be used to generate most of what was previously used and more.

Tablegen was also generating attributes from 2 identical files leading to identical output. so I removed one of them and made user use the other.

Reviewers: jdoerfert, thakis, aaron.ballman

Reviewed By: aaron.ballman

Subscribers: mgorny, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72455