[CMake] Output the error message when get_errc_messages fails

This makes it easier figure out the cause when it fails, and is what
check_z3_version does (the other place we use try_run).

Author: john-brawn-arm

llvm/cmake/modules/GetErrcMessages.cmake

@@ -38,6 +38,7 @@ function(get_errc_messages outvar)
         set(${outvar} ${errc_result} PARENT_SCOPE)
     else()
         set(${outvar} "" PARENT_SCOPE)
+        message(NOTICE "${errc_compile_errors}")
         message(STATUS "Failed to get errc messages")
     endif ()
 endfunction()