gcc/fold-const: remove fold_*_overflow_warnings

fold_undefer_overflow_warnings () is one of the last few places where
Wstrict_overflow warnings are issued.  It uses a
fold_deferring_overflow_warnings int to determine whether the warning should
be shown, a fold_defer_overflow_warnings helper that increments it,
a fold_deferred_overflow_warning string that stores the
deferred warning, an enum for the warning level of the deferred warning.

Remove all of them along with any related logic.

gcc/c-family/ChangeLog:

	* c-common.cc (pointer_int_sum): Removed
	fold_*_overflow_warnings uses.

gcc/c/ChangeLog:

	* c-fold.cc (c_disable_warnings): Likewise.
	(c_enable_warnings): Likewise.

gcc/ChangeLog:

	* flags.h (issue_strict_overflow_warning): Removed.
	* fold-const.cc (fold_defer_overflow_warnings): Removed.
	(fold_undefer_overflow_warnings): Removed.
	(fold_undefer_and_ignore_overflow_warnings): Removed.
	(fold_deferring_overflow_warnings_p): Removed.
	* fold-const.h (fold_defer_overflow_warnings): Removed.
	(fold_undefer_overflow_warnings): Removed.
	(fold_undefer_and_ignore_overflow_warnings): Removed.
	(fold_deferring_overflow_warnings_p): Removed.
	* gimple-fold.cc (fold_stmt_1): Removed
	fold_*_overflow_warnings uses and all related logic.
	* passes.cc (verify_interpass_invariants): Likewise.
	(execute_one_ipa_transform_pass): Likewise.
	(execute_one_pass): Likewise.
	* tree-cfgcleanup.cc (cleanup_control_expr_graph): Likewise.
	* tree-data-ref.cc (create_runtime_alias_checks): Likewise.
	* tree-ssa-ccp.cc (evaluate_stmt): Likewise.
	* tree-ssa-forwprop.cc (combine_cond_expr_cond): Likewise.
	* tree-ssa-loop-niter.cc (expand_simple_operations): Likewise.
	(number_of_iterations_exit_assumptions): Likewise.
	(loop_niter_by_eval): Likewise.
	(estimate_numbers_of_iterations): Likewise.
	(loop_exits_before_overflow): Likewise.
	* tree-ssa-threadedge.cc: Likewise.
	* tree-vrp.cc (compare_values): Likewise.