Make two range_bounds bitfields unsigned

Make two range_bounds bitfields unsigned

While debugging gdb, I noticed that the bitfields in a range_bounds
were signed, causing the values of these fields to be -1.

I think this is odd; and while we haven't yet committed to boolean
bitfields, I think it is a small improvement to change these types to
unsigned.

gdb/ChangeLog
2019-11-28  Tom Tromey  <tom@tromey.com>

	* gdbtypes.h (struct range_bounds) <flag_upper_bound_is_count,
	flag_bound_evaluated>: Now unsigned.

Change-Id: Ia377fd931594bbf8653180d4dcb4e60354d90139