Add a license comment to the generated arm_neon.h header.

Add a license comment to the generated arm_neon.h header.
Remove the previous header.  I don't think we need to expose to end users
that we use TableGen to produce our version of arm_neon.h, and that header
was also using doubleslash comments which could be a problem when using it
in strict C89 compilations.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@121390 91177308-0d34-0410-b5e6-96231b3b80d8