gdb/dwarf: track comp and type units count

A subsequent commit will remove the all_comp_units and all_type_units
array views, since the all_units vector will no longer be segmented
between comp and type units.  Some callers still need to know the number
of each kind, so track that separately.

Change-Id: I6ef184767a96e5be095bbf9142aa850adbb083ac
2 files changed