Suppress uninitialized warnings for new created uses from __builtin_clear_padding...

Suppress uninitialized warnings for new created uses from __builtin_clear_padding folding [PR104550]

__builtin_clear_padding(&object) will clear all the padding bits of the object.
actually, it doesn't involve any use of an user variable. Therefore, users do
not expect any uninitialized warning from it. It's reasonable to suppress
uninitialized warnings for all new created uses from __builtin_clear_padding
folding.

	PR middle-end/104550

gcc/ChangeLog:

	* gimple-fold.cc (clear_padding_flush): Suppress warnings for new
	created uses.

gcc/testsuite/ChangeLog:

	* gcc.dg/auto-init-pr104550-1.c: New test.
	* gcc.dg/auto-init-pr104550-2.c: New test.
	* gcc.dg/auto-init-pr104550-3.c: New test.