dwarf: emit DW_AT_name for DW_TAG_GNU_formal_parameter_pack [PR70536]

dwarf: emit DW_AT_name for DW_TAG_GNU_formal_parameter_pack [PR70536]

Per https://wiki.dwarfstd.org/C++0x_Variadic_templates.md
DW_TAG_GNU_formal_parameter_pack should have a DW_AT_name:

17$:      DW_TAG_formal_parameter_pack
              DW_AT_name("args")
18$:          DW_TAG_formal_parameter
                  ! no DW_AT_name attribute
                  DW_AT_type(reference to 13$)
(...)

	PR c++/70536

gcc/ChangeLog:

	* dwarf2out.cc (gen_formal_parameter_pack_die): Add name attr.

gcc/testsuite/ChangeLog:

	* g++.dg/debug/dwarf2/template-func-params-7.C: Check for pack names.

Co-authored-by: Jason Merrill <jason@redhat.com>