Dummy CUs are used by the incremental linker to pre-allocate space
in the output file. They have a DWARF header but no contents.
gdb/ChangeLog:
* dwarf2read.c (dwarf2_per_cu_data): Add comment.
(load_cu): Handle dummy CUs.
(dw2_do_instantiate_symtab, process_queuef): Ditto.
(dwarf2_fetch_die_loc_sect_off, dwarf2_fetch_constant_bytes): Ditto.
gdb/testsuite/ChangeLog:
* gdb.dwarf2/dw2-dummy-cu.S: New file.
* gdb.dwarf2/dw2-dummy-cu.exp: New file.
