[gdb/symtab] Add all_comp_units/all_type_units views on all_units

Add all_comp_units/all_type_units views on all_units.

Having the views allows us to:
- easily get the number of CUs or TUs in all_units, and
- easily access the nth CU or TU.

This minimizes the use of tu_stats.nr_tus.

Tested on x86_64-linux.