re PR debug/78587 (dwarf2out.c:1517:45: runtime error: negation of -9223372036854775808 cannot be represented in type 'long int [4]'; cast to an unsigned type to negate this value to itself)

	PR debug/78587
	* dwarf2out.c (loc_descr_plus_const): For negative offset use
	uint_loc_descriptor instead of int_loc_descriptor and perform negation
	in unsigned HOST_WIDE_INT type.
	(scompare_loc_descriptor): Shift UINTVAL left instead of INTVAL.

	* gcc.dg/debug/pr78587.c: New test.

From-SVN: r243100