Make clang HIP headers compatible with C++98

Automation to detect compiler features, such as CMake's target_compile_features,
would attempt to detect compiler features by explicitly using langugage flags.
This change ensures that the HIP headers would still work with C++98.

Patch by Siu Chi Chan

Differential Revision: https://reviews.llvm.org/D85471

Change-Id: I304e964b18a525b0fde55efd841da74b6c4dc8ed