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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

What's the suggested way to handle this in a standard build? The documents state to use `-DCMAKE_CXX_FLAGS=--gcc-isntall-dir=<dir>` but that doesn't work if you build LLVM with `gcc` initially. You'd need to somehow only pass that flag to the invocations that use `clang`. We also have some code in the OpenMP project that doesn't respect `CMAKE_CXX_FLAGS` because it needs to be very carefully controlled to generate valid GPU code. I don't think what's suggested anywhere is a valid replacement for the old behavior and is a regression for anyone unfortunate enough to need to build this on a cluster machine where they don't control the global GCC installation.

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