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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@155048 91177308-0d34-0410-b5e6-96231b3b80d8