Revert some Hexagon builtin commits to match reverts done to LLVM in

r155047. See the LLVM log for the primary motivation:
  http://llvm.org/viewvc/llvm-project?rev=155047&view=rev

Primary commit r154828:
  - Several issues were raised in review, and fixed in subsequent
    commits.
  - Follow-up commits also reverted, and which should be folded into the
    original before reposting:
    - r154837: Re-add the 'undef BUILTIN' thing to fix the build.
    - r154928: Fix build warnings, re-add (and correct) header and
      license
    - r154937: Typo fix.

Please resubmit this patch with the relevant LLVM resubmission.

llvm-svn: 155048