c++: Fix pretty-print of pointer minus integer.

c++: Fix pretty-print of pointer minus integer.

For whatever reason, GCC internally represents a pointer minus an integer as
a pointer plus a very large unsigned integer.  But exposing that to users is
unsightly, and it's easy enough to show the real value.

gcc/cp/ChangeLog:

	* error.c (dump_binary_op): Handle negative operand to
	POINTER_PLUS_EXPR.

gcc/c-family/ChangeLog:

	* c-pretty-print.c (pp_c_additive_expression): Handle negative
	operand to POINTER_PLUS_EXPR.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-ptrsub2.C: New test.