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

[Harmen Stoppels via cfe-commits]

haampie wrote:

I disagree, `--gcc-install-dir` is sure an improvement over `--gcc-toolchain`, but they're both weaker than the compile time option `GCC_INSTALL_PREFIX` because of runtimes.

You're looking to remove `GCC_INSTALL_PREFIX`, then give a clear alternative that's equivalent. The current error message about config files is misleading since it potentially runtimes will use an undesired gcc version when they're built, which is rather opaque.

A different point: `flang-new` does not accept `--gcc-install-dir`. Is that an oversight?

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