[OpenMP] Basic support for a parallel directive in a target region on an NVPTX device

Summary:

This patch introduces support for the execution of parallel constructs in a target
region on the NVPTX device.  Parallel regions must be in the lexical scope of the
target directive.

The master thread in the master warp signals parallel work for worker threads in worker
warps on encountering a parallel region.

Note: The patch does not yet support capture of arguments in a parallel region so
the test cases are simple.

Reviewers: ABataev
Differential Revision: https://reviews.llvm.org/D28145

llvm-svn: 291565