Fix PR c++/70452 (regression in C++ parsing performance)

Fix PR c++/70452 (regression in C++ parsing performance)

gcc/cp/ChangeLog:

	PR c++/70452
	* constexpr.c (struct fundef_copy): New struct.
	(struct fundef_copies_table_t): New struct.
	(fundef_copies_table): New static variable.
	(maybe_initialize_fundef_copies_table): New static function.
	(get_fundef_copy): New static function.
	(save_fundef_copy): New static function.
	(cxx_eval_call_expression): Use get_fundef_copy, and
	save_fundef_copy.
	(constexpr_call_table): Add "deletable" GTY marker.

gcc/testsuite/ChangeLog:

	PR c++/70452
	* g++.dg/ext/constexpr-vla4.C: New test.

From-SVN: r234753