Preserve artificial CU name in process_psymtab_comp_unit_reader

Preserve artificial CU name in process_psymtab_comp_unit_reader

This fixes a use-after-free that Simon pointed out.
process_psymtab_comp_unit_reader was allocating an artificial name for
a CU, and then discarding it.  However, this name was preserved in the
cached file_and_directory.  This patch arranges for the allocated name
to be preserved there.