c: fix uninitialized c_expr::m_decimal [PR106830]

c: fix uninitialized c_expr::m_decimal [PR106830]

I added c_expr::m_decimal in r13-2386-gbedfca647a9e9c1a as part of the
implementation of -Wxor-used-as-pow, but I missed various places where
the field needed to be initialized.

Fixed thusly.

gcc/c-family/ChangeLog:
	PR c/106830
	* c-warn.cc (check_for_xor_used_as_pow): Don't try checking
	values that don't fit in uhwi.

gcc/c/ChangeLog:
	PR c/106830
	* c-parser.cc (c_parser_initelt): Initialize m_decimal.
	(c_parser_cast_expression): Likewise.
	(c_parser_alignof_expression): Likewise.
	(c_parser_postfix_expression_after_paren_type): Likewise.
	(c_parser_postfix_expression_after_primary): Likewise.
	(c_parser_expression): Likewise.
	(c_parser_omp_variable_list): Likewise.
	(c_parser_transaction_expression): Likewise.
	* c-tree.h (c_expr::set_error): Likewise.
	* c-typeck.cc (c_expr_sizeof_expr): Likewise.
	(parser_build_unary_op): Likewise.
	(parser_build_binary_op): Likewise.
	(digest_init): Likewise.
	(pop_init_level): Likewise.
	* gimple-parser.cc (c_parser_gimple_call_internal): Likewise.

gcc/testsuite/ChangeLog:
	PR c/106830
	* gcc.dg/Wxor-used-as-pow-pr106830.c: New test.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>