Pass `-g` to enable debugging, not `-O` for optimizations. This was
probably a typo in commit b50bfc8913 (HIP: Add language to CMake,
2020-08-28, v3.21.0-rc1~66^2~4).
Fixes: #26823
It only makes sense to use the CMake package from the same ROCm
installation that the compiler uses. Ask the HIP compiler to report the
location of the ROCm installation. Verify up front that it contains the
expected CMake package file.