Since commit bd844387df (ROCMClang: Add the ROCm toolkit derived clang
compiler to CMake, 2020-08-28, v3.21.0-rc1~66^2~6) and commit ff0d2858e1
(HIP: Extract clang compiler details from hipcc, 2020-10-21,
v3.21.0-rc1~66^2~5), the separate `ROCMClang` compiler id for `hipcc`
has caused a few problems:
* The compiler id changed from behavior of CMake 3.20 and below,
breaking projects that already built with `hipcc` treated as `Clang`.
* The implementation of `target_compile_features` was incomplete for
the `ROCMClang` identity.
* Only `hipcc` was identified as `ROCMClang`, so after it is unwrapped
to the underlying `clang++`, future runs of new CMake versions on
an existing build tree would not repeat this.
* Clang should be usable as a HIP compiler without the `hipcc` wrapper.
Remove the `ROMClang` compiler identity, and revise HIP language support
to work directly with a Clang compiler.
Reject direct `hipcc` usage as a HIP compiler. For now it cannot be
supported because it interferes with flags CMake needs to pass to Clang.
Fixes: #22536, #22460, #22593
Seems the relative paths were wrong basically all around such that only
compiling files in the top-level directory would work. I've modified
CudaOnly.SeparateCompilation to cover this.
Fixes#22482.
