c++: Avoid calls in non-evaluated contexts affect whether function can or can't throw...

c++: Avoid calls in non-evaluated contexts affect whether function can or can't throw [PR94326]

The following testcase FAILs -fcompare-debug, because if we emit a
-Wreturn-local-addr warning, we tsubst decltype in order to print the
warning and as that function could throw, set_flags_from_callee during that
sets cp_function_chain->can_throw and later on we don't set TREE_NOTHROW
on foo.  While with -w or -Wno-return-local-addr, tsubst isn't called during
the warning_at, cp_function_chain->can_throw is kept clear and TREE_NOTHROW
is set on foo.
It isn't just a matter of the warning though, in
int foo ();
int bar () { return sizeof (foo ()); }
int baz () { return sizeof (int); }
I don't really see why we should mark only baz as TREE_NOTHROW and not bar
too, when neither can really throw.

2020-03-27  Jakub Jelinek  <jakub@redhat.com>

	PR c++/94326
	* call.c (set_flags_from_callee): Don't update
	cp_function_chain->can_throw or current_function_returns_abnormally
	if cp_unevaluated_operand.

	* g++.dg/other/pr94326.C: New test.