Fix handling of DW_TAG_unspecified_type for Ada

Fix handling of DW_TAG_unspecified_type for Ada

Commit 80eaec735e ("[gdb/symtab] Handle named DW_TAG_unspecified_type
DIE") changed the handling of DW_TAG_unspecified_type.  Before this
change, such types were not entered into the symbol table.

It turns out that, when such a type is in the symtab, it can cause
failures in Ada.  In particular, a private type in another package may
be seen locally as "void".

Now, it would probably be better to fix this via check_typedef.
However, that is somewhat difficult given the state of the DWARF
reader -- in particular with gdb_index, this would require expanding
potentially many CUs to find the correct type.

Instead, this patch changes gdb to not enter a symbol for an
unspecified type -- but only for Ada.