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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> `-DRUNTIMES_CMAKE_ARGS="-DCMAKE_C_FLAGS=--gcc-install-dir=$GCC_ROOT;-DCMAKE_CXX_FLAGS=--gcc-install-dir=$GCC_ROOT"` worked for me

Great, we should probably document this somewhere.

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