c++: Readd [LR]ROTATE_EXPR support to constexpr.c [PR94067]

c++: Readd [LR]ROTATE_EXPR support to constexpr.c [PR94067]

Since r10-6527-gaaa26bf496a646778ac861aed124d960b5bf549f fold_for_warn
will perform maybe_constant_value even on some cp_fold produced trees and
so can include rotate exprs which were removed last fall from constexpr.c

2020-03-09  Jakub Jelinek  <jakub@redhat.com>

	PR c++/94067
	Revert
	2019-10-11  Paolo Carlini  <paolo.carlini@oracle.com>

	* constexpr.c (cxx_eval_constant_expression): Do not handle
	RROTATE_EXPR and LROTATE_EXPR.

	* g++.dg/warn/Wconversion-pr94067.C: New test.