c++: Fix wrong-code with non-constexpr constructor [PR93169]

c++: Fix wrong-code with non-constexpr constructor [PR93169]

In order to detect modifying constant objects in constexpr evaluation,
which is UB, in r10-2655 I added code that sets TREE_READONLY on
CONSTRUCTORs of const-qualified objects after they have been fully
constructed.  But I never made sure that what we're setting the flag
on actually is a CONSTRUCTOR.  Consequently, as this test case shows,
we could set TREE_READONLY on a VAR_DECL that in fact wasn't constant,
causing problems later.  Fixed by setting the flag on CONSTRUCTORs
only, and only when the evaluation produced something constant.

2020-02-19  Marek Polacek  <polacek@redhat.com>

	PR c++/93169 - wrong-code with a non-constexpr constructor.
	* constexpr.c (cxx_eval_call_expression): Only set TREE_READONLY
	on constant CONSTRUCTORs.

	* g++.dg/cpp0x/constexpr-93169.C: New test.