[clang] [HIP] Fix return type in __clang_hip_cmath.h (PR #139697)

[Juan Manuel Martinez Caamaño via cfe-commits]

jmmartinez wrote:

Added a tests and created a separate PR to commit the tests before the fix in https://github.com/llvm/llvm-project/pull/139891

https://github.com/llvm/llvm-project/pull/139697