[Clang][SYCL][NFC] Modify err_sycl_entry_point_invalid to use %enum_select. (#173122)
The `err_sycl_entry_point_invalid` diagnostic has a selection field for
which there are already many options with more expected to be added. Use
of `%enum_select` avoids the need for magic numbers with associated
comments at source locations where the diagnostic is issued.