Fix latent bug in dwarf2_find_containing_comp_unit

Fix latent bug in dwarf2_find_containing_comp_unit

dwarf2_find_containing_comp_unit has this in its binary search:

      if (mid_cu->is_dwz > offset_in_dwz
	  || (mid_cu->is_dwz == offset_in_dwz
	      && mid_cu->sect_off + mid_cu->length >= sect_off))
	high = mid;

The intent here is to determine whether SECT_OFF appears in or before
MID_CU.

I believe this has an off-by-one error, and that the check should use
">" rather than ">=".  If the two side are equal, then SECT_OFF
actually appears at the start of the next CU.

I've had this patch kicking around for ages but I forget how I found
the problem.

gdb/ChangeLog
2020-02-20  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.c (dwarf2_find_containing_comp_unit): Use ">", not
	">=", in binary search.
	(dwarf2_find_containing_comp_unit): New overload.
	(run_test): New self-test.
	(_initialize_dwarf2_read): Register new test.