[UnrollAndJam] Add unroll_and_jam pragma handling

This adds support for the unroll_and_jam pragma, to go with the recently
added unroll and jam pass. The name of the pragma is the same as is used
in the Intel compiler, and most of the code works the same as for unroll.

#pragma clang loop unroll_and_jam has been separated into a different
patch. This part adds #pragma unroll_and_jam with an optional count, and
#pragma no_unroll_and_jam to disable the transform.

Differential Revision: https://reviews.llvm.org/D47267

llvm-svn: 338566