gdb: split get_discrete_bounds in two

gdb: split get_discrete_bounds in two

get_discrete_bounds is not flexible for ranges (TYPE_CODE_RANGE), in the
sense that it returns true (success) only if both bounds are present and
constant values.

This is a problem for code that only needs to know the low bound and
fails unnecessarily if the high bound is unknown.

Split the function in two, get_discrete_low_bound and
get_discrete_high_bound, that both return an optional.  Provide a new
implementation of get_discrete_bounds based on the two others, so the
callers don't have to be changed.

gdb/ChangeLog:

	* gdbtypes.c (get_discrete_bounds): Implement with
	get_discrete_low_bound and get_discrete_high_bound.
	(get_discrete_low_bound): New.
	(get_discrete_high_bound): New.

Change-Id: I986b5e9c0dd969800e3fb9546af9c827d52e80d0