[HIP] Use native math functions for -fcuda-approx-transcendentals

CUDA allows replacing standard math functions with less accurate
native math functions when -use_fast_math is specified
(https://docs.nvidia.com/cuda/cuda-c-programming-guide/index.html#intrinsic-functions).
Cuda-clang does this when -fcuda-approx-transcendentals
or -ffast-math is specified. HIP-clang currently passes option
-fcuda-approx-transcendentals to clang -cc1 and predefines
__CLANG_CUDA_APPROX_TRANSCENDENTALS__ but does
not replace standard math functions with native math functions.

This patch implements this in a similar approach as cuda-clang.

Reviewed by: Brian Sumner, Matt Arsenault

Differential Revision: https://reviews.llvm.org/D154790