Since commit bd844387 (ROCMClang: Add the ROCm toolkit derived clang
compiler to CMake, 2020-08-28, v3.21.0-rc1~66^2~6) and commit ff0d2858
(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.  Fo...