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

unless USE_DEPRECATED_GCC_INSTALL_PREFIX (temporary escape hatch) is
set. Setting GCC_INSTALL_PREFIX leads to a warning for Clang 18.1
(#77537) and will be completely removed for Clang 20.

Link:
discourse.llvm.org/t/add-gcc-install-dir-deprecate-gcc-toolchain-and-remove-gcc-install-prefix/65091
Link:
discourse.llvm.org/t/correct-cmake-parameters-for-building-clang-and-lld-for-riscv/72833

Author: MaskRay

clang/CMakeLists.txt

@@ -190,11 +190,12 @@ set(CLANG_RESOURCE_DIR "" CACHE STRING
 set(C_INCLUDE_DIRS "" CACHE STRING
   "Colon separated list of directories clang will search for headers.")
 
+set(USE_DEPRECATED_GCC_INSTALL_PREFIX OFF CACHE BOOL "Temporary workaround before GCC_INSTALL_PREFIX is completely removed")
 set(GCC_INSTALL_PREFIX "" CACHE PATH "Directory where gcc is installed." )
 set(DEFAULT_SYSROOT "" CACHE STRING
   "Default <path> to all compiler invocations for --sysroot=<path>." )
-if(GCC_INSTALL_PREFIX)
-  message(WARNING "GCC_INSTALL_PREFIX is deprecated and will be removed. Use "
+if(GCC_INSTALL_PREFIX AND NOT USE_DEPRECATED_GCC_INSTALL_PREFIX)
+  message(FATAL_ERROR "GCC_INSTALL_PREFIX is deprecated and will be removed. Use "
     "configuration files (https://clang.llvm.org/docs/UsersManual.html#configuration-files)"
     "to specify the default --gcc-install-dir= or --gcc-triple=. --gcc-toolchain= is discouraged. "
     "See https://github.com/llvm/llvm-project/pull/77537 for detail.")

clang/docs/ReleaseNotes.rst

@@ -37,6 +37,9 @@ These changes are ones which we think may surprise users when upgrading to
 Clang |release| because of the opportunity they pose for disruption to existing
 code bases.
 
+- Setting the deprecated CMake variable ``GCC_INSTALL_PREFIX`` (which sets the
+  default ``--gcc-toolchain=``) now leads to a fatal error.
+
 C/C++ Language Potentially Breaking Changes
 -------------------------------------------