[HIP] Restructure hip headers to add cmath

Separate __clang_hip_math.h header into __clang_hip_cmath.h
and __clang_hip_math.h. Improve the math function definition,
and add missing definitions or declarations. Add missing
overloads.

Reviewed By: tra, JonChesterfield

Differential Review: https://reviews.llvm.org/D88837