c++: Fix up handling of name independent structured binding packs [PR117783]

c++: Fix up handling of name independent structured binding packs [PR117783]

I've realized I haven't added testsuite coverage for name independent
structured binding packs.  And the
  auto [i, ..._, j] = T {};
  auto [k, ..._, l] = T {};
case shows a problem with that.  The elements of the structured binding
pack have #i appended to their names, so for the _ case e.g. _#0, _#1
etc. (to print something useful in diagnostics, perhaps debug info later
on).  The above is valid though as long as one doesn't use _ (which is
ambiguous), but we were emitting errors on redeclaration of _#0, _#1
etc.

The following patch uses DECL_NAME (decl) = NULL_TREE; for the
name independent decl case so that the false positive redeclaration
errors aren't emitted.

2025-08-11  Jakub Jelinek  <jakub@redhat.com>

	PR c++/117783
	* decl.cc (set_sb_pack_name): For name independent decls
	just clear DECL_NAME instead of appending #i to it.

	* g++.dg/cpp26/name-independent-decl11.C: New test.