[Test] Invoke GetErrcMessages from config-ix instead of llvm/CMakeLists

Since GetErrcMessages uses cmake's `try_run` mechanism it's sensitive
to changes to CMAKE_CXX_FLAGS, so we move it into config-ix with the
similar flag-sensitive configuration. This makes it run before
HandleLLVMOptions and avoids issues with LLVM_ENABLE_WERROR and other
configuration that manipulate CMAKE_CXX_FLAGS.

Differential Revision: https://reviews.llvm.org/D144526