Fix DWARFUnitSection::getUnitForOffset().

The current code is only able to return the right unit if the passed offset
is the exact offset of a section. Generalize the search function by comparing
againt the offset of the next unit instead and by switching the search
algorithm to upper_bound.

This way, the unit returned is the first unit with a getNextUnitOffset()
strictly greater than the searched offset, which is exactly what we want.
Note that there is no need for testing the range of the resulting unit as
the offsets of a DWARFUnitSection are in a single contiguous range from
0 inclusive to lastUnit->getNextUnitOffset() exclusive.

Reviewers: dblaikie samsonov

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5262