[clang] [CMake] Change GCC_INSTALL_PREFIX from warning to fatal error (PR #85891)

[Harmen Stoppels via cfe-commits]

haampie wrote:

Users of `GCC_INSTALL_PREFIX` typically have GCC installed in a unique, non-standard prefix, where clang's default search just worked.

Now those users have to generate config files for post-install, AND figure out how to make clang behave correctly when building runtimes. That's not ideal.

@MaskRay can you please make the error message more informative? It should probably say that users additionally have to set `RUNTIMES_CMAKE_ARGS=...` as well as generating `clang.cfg`, `clang++.cfg` config files. (And what about `flang.cfg` and/or `flang-new.cfg`?)

More clarity please :)

https://github.com/llvm/llvm-project/pull/85891