c++: constantness of local var in constexpr fn [PR111703, PR112269]

c++: constantness of local var in constexpr fn [PR111703, PR112269]

potential_constant_expression was incorrectly treating most local
variables from a constexpr function as constant because it wasn't
considering the 'now' parameter.  This patch fixes this by relaxing
its var_in_maybe_constexpr_fn checks accordingly, which turns out to
partially fix two recently reported regressions:

PR111703 is a regression caused by r11-550-gf65a3299a521a4 for restricting
constexpr evaluation during warning-dependent folding.  The mechanism is
intended to restrict only constant evaluation of the instantiated
non-dependent expression, but it also ends up restricting constant
evaluation occurring during instantiation of the expression, in particular
when instantiating the converted argument 'x' (a VIEW_CONVERT_EXPR) into
a copy constructor call.  This seems like a flaw in the mechanism, though
I don't know if we want to fix the mechanism or get rid of it completely
since the original testcases which motivated the mechanism are fixed more
simply by r13-1225-gb00b95198e6720.  In any case, this patch partially
fixes this by making us correctly treat 'x' as non-constant which prevents
the problematic warning-dependent folding from occurring at all.

PR112269 is caused by r14-4796-g3e3d73ed5e85e7 for merging tsubst_copy
into tsubst_copy_and_build.  tsubst_copy used to exit early when 'args'
was empty, behavior which that commit deliberately didn't preserve.
This early exit masked the fact that COMPLEX_EXPR wasn't handled by
tsubst at all, and is a tree code that apparently we could see during
warning-dependent folding on some targets.  A complete fix is to add
handling for this tree code in tsubst_expr, but this patch should fix
the reported testsuite failures since the COMPLEX_EXPRs that crop up
in <complex> are considered non-constant expressions after this patch.

	PR c++/111703
	PR c++/112269

gcc/cp/ChangeLog:

	* constexpr.cc (potential_constant_expression_1) <case VAR_DECL>:
	Only consider var_in_maybe_constexpr_fn if 'now' is false.
	<case INDIRECT_REF>: Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-fn8.C: New test.