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

[bogner]

Summary

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.

[beanz]

LGTM

llvm/CMakeLists.txt

configure_file(
   ${LLVM_INCLUDE_DIR}/llvm/Config/TargetExegesis.def
   )
 
 # They are not referenced. See set_output_directory().
 set( CMAKE_RUNTIME_OUTPUT_DIRECTORY ${LLVM_TOOLS_BINARY_DIR} )
 set( CMAKE_LIBRARY_OUTPUT_DIRECTORY ${LLVM_LIBRARY_DIR} )
 set( CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${LLVM_LIBRARY_DIR} )
 
-if(LLVM_INCLUDE_TESTS)
-  include(GetErrcMessages)
-  get_errc_messages(LLVM_LIT_ERRC_MESSAGES)
-endif()
-
 # For up-to-date instructions for installing the TFLite dependency, refer to
 # the bot setup script: https://github.com/google/ml-compiler-opt/blob/main/buildbot/buildbot_init.sh
 set(LLVM_HAVE_TFLITE "" CACHE BOOL "Use tflite")
 if (LLVM_HAVE_TFLITE)
   find_package(tensorflow-lite REQUIRED)
 endif()
 
 # For up-to-date instructions for installing the Tensorflow dependency, refer to

llvm/cmake/config-ix.cmake

else()
     # Only recent versions of gcc make the distinction between -Wuninitialized
     # and -Wmaybe-uninitialized. If -Wmaybe-uninitialized isn't supported, just
     # turn off all uninitialized use warnings.
     check_cxx_compiler_flag("-Wuninitialized" HAS_UNINITIALIZED)
     set(USE_NO_UNINITIALIZED ${HAS_UNINITIALIZED})
   endif()
 endif()
 
+if(LLVM_INCLUDE_TESTS)
+  include(GetErrcMessages)
+  get_errc_messages(LLVM_LIT_ERRC_MESSAGES)
+endif()
+
 # By default, we target the host, but this can be overridden at CMake
 # invocation time.
 include(GetHostTriple)
 get_host_triple(LLVM_INFERRED_HOST_TRIPLE)
 
 set(LLVM_HOST_TRIPLE "${LLVM_INFERRED_HOST_TRIPLE}" CACHE STRING
     "Host on which LLVM binaries will run")