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

[Ye Luo via cfe-commits]

ye-luo wrote:

The term deprecated refers to a functionality that still exists in software whose use is not recommended. Contradictorily, GCC_INSTALL_PREFIX got me fatal error now without a clear message explaining how to keep it working. I had to dig deep to figure it out.

Will "temporary" `USE_DEPRECATED_GCC_INSTALL_PREFIX` make its way to 19 release? Then it should be documented.

Why `USE_DEPRECATED_GCC_INSTALL_PREFIX` is even needed in the first place? It adds further complexity. Why not just delete `GCC_INSTALL_PREFIX`?

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