[gdb/symtab] Make gold index workaround more precise

There's a PR gold/15646 - "gold-generated .gdb_index has duplicated
symbols that gdb-generated index doesn't", that causes gold to generate
duplicate symbols in the index.

F.i., a namespace N1 declared in a header file can be listed for two CUs that
include the header file:
...
[759] N1:
        2 [global type]
        3 [global type]
...

This causes a gdb performance problem: f.i. when attempting to set a
breakpoint on a non-existing function N1::misspelled, the symtab for both CUs
will be expanded.

Gdb contains a workaround for this, added in commit 8943b874 "Work around
gold/15646", that skips duplicate global symbols in the index.

However, the workaround does not check for the symbol kind ("type" in the
example above).

Make the workaround more precise by limiting it to symbol kind "type".

Tested on x86_64-linux, with target boards cc-with-gdb-index and
gold-gdb-index.

gdb/ChangeLog:

2020-05-28  Tom de Vries  <tdevries@suse.de>

	* dwarf2/read.c	(dw2_symtab_iter_next, dw2_expand_marked_cus): Limit
	PR gold/15646 workaround to symbol kind "type".