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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

@petrhosek Is there a way to pass flags only to the runtimes portion of the build within the normal workflow? I know we have `-DRUNTIMES_x86_64-unknown-linux-gnu_CMAKE_CXX_COMPILE_FLAGS=` that might work, but I don't think this is respected if we're using the `default` target.

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