c++: Fix ICE with nsdmi [PR99705]

c++: Fix ICE with nsdmi [PR99705]

When adding P0784R7 constexpr new support, we still didn't have
P1331R2 implemented and so I had to change also build_vec_delete_1
- instead of having uninitialized tbase temporary later initialized
by MODIFY_EXPR I've set the DECL_INITIAL for it - because otherwise
it would be rejected during constexpr evaluation which didn't like
uninitialized vars.  Unfortunately, that change broke the following
testcase.
The problem is that these temporaries (not just tbase but tbase was
the only one with an initializer) are created during NSDMI parsing
and current_function_decl is NULL at that point.  Later when we
clone body of constructors, auto_var_in_fn_p is false for those
(as they have NULL DECL_CONTEXT) and so they aren't duplicated,
and what is worse, the DECL_INITIAL isn't duplicated either nor processed,
and during expansion we ICE because the code from DECL_INITIAL of that
var refers to the abstract constructor's PARM_DECL (this) rather than
the actual constructor's one.

So, either we can just revert those build_vec_delete_1 changes (as done
in the second patch - in attachment), or, as the first patch does, we can
copy the temporaries during bot_manip like we copy the temporaries of
TARGET_EXPRs.  To me that looks like a better fix because e.g. if
break_out_of_target_exprs is called for the same NSDMI multiple times,
sharing the temporaries looks just wrong to me.  If the temporaries
are declared as BIND_EXPR_VARS of some BIND_EXPR (which is the case
of the tbase variable built by build_vec_delete_1 and is the only way
how the DECL_INITIAL can be walked by *walk_tree*), then we need to
copy it also in the BIND_EXPR BIND_EXPR_VARS chain, other temporaries
(those that don't need DECL_INITIAL) often have just DECL_EXPR and no
corresponding BIND_EXPR.
Note, ({ }) are rejected in nsdmis, so all we run into are temporaries
the FE creates artificially.

2021-03-26  Jakub Jelinek  <jakub@redhat.com>

	PR c++/99705
	* tree.c (bot_manip): Remap artificial automatic temporaries mentioned
	in DECL_EXPR or in BIND_EXPR_VARS.

	* g++.dg/cpp0x/new5.C: New test.