libcpp: Fix unsigned promotion for unevaluated divide by zero [PR112701]

When libcpp encounters a divide by zero while processing a constant
expression "x/y", it returns "x" as a fallback. The value of the fallback is
not normally important, since an error will be generated anyway, but if the
expression appears in an unevaluated context, such as "0 ? 0/0u : -1", then
there will be no error, and the fallback value will be meaningful to the
extent that it may cause promotion from signed to unsigned of an operand
encountered later. As the PR notes, libcpp does not do the unsigned
promotion correctly in this case; fix it by making the fallback return value
unsigned as necessary.

libcpp/ChangeLog:

	PR preprocessor/112701
	* expr.cc (num_div_op): Set unsignedp appropriately when returning a
	stub value for divide by 0.

gcc/testsuite/ChangeLog:

	PR preprocessor/112701
	* gcc.dg/cpp/expr.c: Add additional tests to cover divide by 0 in an
	unevaluated context, where the unsignedness still matters.