c++: quadratic constexpr behavior for left-assoc logical exprs [PR102780]

c++: quadratic constexpr behavior for left-assoc logical exprs [PR102780]

In the testcase below the two left fold expressions each expand into a
constant logical expression with 1024 terms, for which potential_const_expr
takes more than a minute to return true.  This happens because p_c_e_1
performs trial evaluation of the first operand of a &&/|| in order to
determine whether to consider the potentiality of the second operand.
And because the expanded expression is left-associated, this trial
evaluation causes p_c_e_1 to be quadratic in the number of terms of the
expression.

This patch fixes this quadratic behavior by making p_c_e_1 preemptively
compute potentiality of the second operand of a &&/||, and perform trial
evaluation of the first operand only if the second operand isn't
potentially constant.  We must be careful to avoid emitting bogus
diagnostics during the preemptive computation; to that end, we perform
this shortcut only when tf_error is cleared, and when tf_error is set we
now first check potentiality of the whole expression quietly and replay
the check noisily for diagnostics.

Apart from fixing the quadraticness for left-associated logical exprs,
this change also reduces compile time for the libstdc++ testcase
20_util/variant/87619.cc by about 15% even though our <variant> uses
right folds instead of left folds.  Likewise for the testcase in the PR,
for which compile time is reduced by 30%.  The reason for these speedups
is that p_c_e_1 no longer performs expensive trial evaluation of each term
of large constant logical expressions when determining their potentiality.

	PR c++/102780

gcc/cp/ChangeLog:

	* constexpr.c (potential_constant_expression_1) <case TRUTH_*_EXPR>:
	When tf_error isn't set, preemptively check potentiality of the
	second operand before performing trial evaluation of the first
	operand.
	(potential_constant_expression_1): When tf_error is set, first check
	potentiality quietly and return true if successful, otherwise
	proceed noisily to give errors.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/fold13.C: New test.