c++: P2280R4 and speculative constexpr folding [PR119387]

c++: P2280R4 and speculative constexpr folding [PR119387]

Compiling the testcase in this PR uses 2.5x more memory and 6x more
time ever since r14-5979 which implements P2280R4.  This is because
our speculative constexpr folding now does a lot more work trying to
fold ultimately non-constant calls to constexpr functions, and in turn
produces a lot of garbage.  We do sometimes successfully fold more
thanks to P2280R4, but it seems to be trivial stuff like calls to
std::array::size or std::addressof.  The benefit of P2280 therefore
doesn't seem worth the cost during speculative constexpr folding, so
this patch restricts the paper to only manifestly-constant evaluation.

	PR c++/119387

gcc/cp/ChangeLog:

	* constexpr.cc (p2280_active_p): New.
	(cxx_eval_constant_expression) <case VAR_DECL>: Use it to
	restrict P2280 relaxations.
	<case PARM_DECL>: Likewise.

Reviewed-by: Jason Merrill <jason@redhat.com>