[HIP] Fix HIP include path

The clang compiler prepends the HIP header include paths to the search
list using -internal-isystem when building for the HIP language. This
prevents warnings related to things like reserved identifiers when
including the HIP headers even when ROCm is installed in a non-system
directory, such as /opt/rocm.

However, when HIP is installed in /usr, then the prepended include
path would be /usr/include. That is a problem, because the C standard
library headers are stored in /usr/include and the C++ standard
library headers must come before the C library headers in the search
path list (because the C++ standard library headers use #include_next
to include the C standard library headers).

While the HIP wrapper headers _do_ need to be earlier in the search
than the C++ headers, those headers get their own subdirectory and
their own explicit -internal-isystem argument. This include path is for
<hip/hip_runtime_api.h> and <hip/hip_runtime.h>, which do not require a
particular search ordering with respect to the C or C++ headers. Thus,
HIP include path is added after other system include paths.

With contribution from Cordell Bloor.

Reviewed by: Artem Belevich

Differential Revision: https://reviews.llvm.org/D120132