[Driver] Override default location of config files

If directory for config files was specified in project configuration
using parameters CLANG_CONFIG_FILE_SYSTEM_DIR or CLANG_CONFIG_FILE_USER_DIR,
it was not overriden by command-line option `--config-system-dir=` or
`--config-user-dir=` that specified empty path.

This change corrects the behavior. It fixes the issue
https://github.com/llvm/llvm-project/issues/56836 ([clang] [test]
test/Driver/config-file-errs.c fails if CLANG_CONFIG_FILE_SYSTEM_DIR is
specified).