c++: non-empty constexpr constructor bodies in C++11 [PR123845]

This patch makes us support C++14 non-empty constexpr constructor bodies
in C++11, as an extension.  This will make it trivial to safely fix the
C++11 library regression PR114865 that requires us to do
__builtin_clear_padding after initializing _M_i in std::atomic's
single-parameter constructor, and that's not really possible with the
C++11 constexpr restrictions.

Since we lower member initializers to constructor body statements
internally, and so constructor bodies are already effectively non-empty
internally even in C++11, supporting non-empty bodies in user code is
mostly a matter of relaxing the parse-time error.

But constexpr-ex3.C revealed that by accepting the non-empty body of A's
constructor, build_data_member_initialization goes on to mistake the
'i = _i' assignment as a member initializer, and we incorrectly accept
the constructor in C++11 mode (even though omitting mem-inits is only
valid since C++20).  Turns out this is caused by that function
recognizing MODIFY_EXPR only in C++11 mode, logic that was last changed
by r5-5013 (presumably to limit impact of the patch at the time) but I
reckon could just be removed outright.  This should be safe because the
result of build_data_member_initialization is only used by
cx_check_missing_mem_inits for validation; evaluation is in terms of
the entire lowered constructor body.

	PR c++/123845
	PR libstdc++/114865

gcc/cp/ChangeLog:

	* constexpr.cc (build_data_member_initialization): Remove
	C++11-specific recognition of MODIFY_EXPR.
	(check_constexpr_ctor_body): Relax error diagnostic to a
	pedwarn and don't clear DECL_DECLARED_CONSTEXPR_P upon
	error.  Return true if complaining.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-ex3.C: Adjust C++11 non-empty
	constexpr constructor dg-error to a dg-warning.  Expect
	a follow-up missing member initializer diagnostic in C++11 mode.
	* g++.dg/cpp2a/constexpr-try1.C: Expect a follow-up
	compound-statement in constexpr function diagnostic in C++11
	mode.
	* g++.dg/cpp2a/constexpr-try2.C: Likewise.  Adjust C++11
	non-empty constexpr constructor dg-error to a dg-warning.
	* g++.dg/cpp2a/constexpr-try3.C:  Adjust C++11 non-empty
	constexpr constructor dg-error to a dg-warning.
	* g++.dg/cpp0x/constexpr-ctor23.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>
(cherry picked from commit e4c57e146a224d0aaa71ace78f96fca1156add24)