[libclang] Expose aligned() attribute.

[libclang] Expose aligned() attribute.

Summary:
This is useful because otherwise there's no easy way to distinguish #pragma
packed(N) from attribute(packed, aligned(N)) that isn't looking at field
offsets (since pragma packed() also creates a packed attribute).

Reviewers: Anastasia, arphaman, serge-sans-paille

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59299

llvm-svn: 356062