[ARM NEON] Avoid duplicated decarations

Summary:
The declaration of arm neon intrinsics that are
"big endian safe" print the same code for big
and small endian targets.
This patch avoids duplicates by checking if an
intrinsic is safe to have a single definition.
(decreases header 11k lines out of 73k).

Reviewers: t.p.northover, ostannard, labrinea

Reviewed By: ostannard

Subscribers: kristof.beyls, cfe-commits, olista01

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66588

llvm-svn: 370716