c/111468 - add unordered compare and pointer diff to GIMPLE FE parsing

The following adds __UN{LT,LE,GT,GE,EQ}, __UNORDERED and __ORDERED
operator parsing support and support for parsing - as POINTER_DIFF_EXPR.

	PR c/111468
gcc/c/
	* gimple-parser.cc (c_parser_gimple_binary_expression): Add
	return type argument.
	(c_parser_gimple_statement): Adjust.
	(c_parser_gimple_paren_condition): Likewise.
	(c_parser_gimple_binary_expression): Use passed in return type,
	add support for - as POINTER_DIFF_EXPR, __UN{LT,LE,GT,GE,EQ},
	__UNORDERED and __ORDERED.

gcc/testsuite/
	* gcc.dg/gimplefe-50.c: New testcase.
	* gcc.dg/gimplefe-51.c: Likewise.