middle-end: Small __builtin_clear_padding improvements

When looking at __builtin_clear_padding today, I've noticed that
it is quite wasteful to extend the original user one argument to 3,
2 is enough.  We need to encode the original type of the first argument
because pointer conversions are useless in GIMPLE, and we need to record
a boolean whether it is for -ftrivial-auto-var-init=* or not.
But for recording the type we don't need the value (we've always used
zero) and for recording the boolean we don't need the type (we've always
used integer_type_node).
So, this patch merges the two into one.

2022-02-11  Jakub Jelinek  <jakub@redhat.com>

	* tree.cc (build_common_builtin_nodes): Fix up formatting in
	__builtin_clear_padding decl creation.
	* gimplify.cc (gimple_add_padding_init_for_auto_var): Encode
	for_auto_init in the value of 2nd BUILT_IN_CLEAR_PADDING
	argument rather than in 3rd argument.
	(gimplify_call_expr): Likewise.  Fix up comment formatting.
	* gimple-fold.cc (gimple_fold_builtin_clear_padding): Expect
	2 arguments instead of 3, take for_auto_init from the value
	of 2nd argument.