[OpenMP] Update target codegen for NVPTX device.

This patch includes updates for codegen of the target region for the NVPTX
device. It moves initializers from the compiler to the runtime and updates
the worker loop to assume parallel work is retrieved from the runtime. A
subsequent patch will update the codegen to retrieve the parallel work using
calls to the runtime. It includes the removal of the inline attribute
for the worker loop and disabling debug info in it.

This allows codegen for a target directive and serial execution on the
NVPTX device.

Reviewers: ABataev
Differential Revision: https://reviews.llvm.org/D28125

llvm-svn: 291121