c++: Fix up handling of structured bindings in extract_locals_r [PR99833]

c++: Fix up handling of structured bindings in extract_locals_r [PR99833]

The following testcase ICEs in tsubst_decomp_names because the assumptions
that the structured binding artificial var is followed in DECL_CHAIN by
the corresponding structured binding vars is violated.
I've tracked it to extract_locals* which is done for the constexpr
IF_STMT.  extract_locals_r when it sees a DECL_EXPR adds that decl
into a hash set so that such decls aren't returned from extract_locals*,
but in the case of a structured binding that just means the artificial var
and not the vars corresponding to structured binding identifiers.
The following patch fixes it by pushing not just the artificial var
for structured bindings but also the other vars.

2021-04-16  Jakub Jelinek  <jakub@redhat.com>

	PR c++/99833
	* pt.c (extract_locals_r): When handling DECL_EXPR of a structured
	binding, add to data.internal also all corresponding structured
	binding decls.

	* g++.dg/cpp1z/pr99833.C: New test.

(cherry picked from commit 06d50ebc9fb2761ed2bdda5e76adb4d47a8ca983)