[OpenMP] AlwaysInline __kmpc_parallel_51 to improve inlining hueristics

This patch adds the `AlwaysInline` attribute to the `__kmpc_parallel_51`
device runtime call. This improves inlining heuristics which encourages
the indirect function pointer arguemnt to also be inlined. This greatly
improves performance for a few applications whose outlined regions were
not inlined otherwise.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D107839