[OpenMP][Clang] Support for target math functions

Summary:
In this patch we propose a temporary solution to resolving math functions for the NVPTX toolchain, temporary until OpenMP variant is supported by Clang.

We intercept the inclusion of math.h and cmath headers and if we are in the OpenMP-NVPTX case, we re-use CUDA's math function resolution mechanism.

Authors:
@gtbercea
@jdoerfert

Reviewers: hfinkel, caomhin, ABataev, tra

Reviewed By: hfinkel, ABataev, tra

Subscribers: JDevlieghere, mgorny, guansong, cfe-commits, jdoerfert

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61399

llvm-svn: 360265