debug/dwarf: factor finding unit containing entry offset
This factors out the code for finding which unit contains an offset in
the "info" section. The new code also replaces linear search with a
binary search. The line table reader will also need this
functionality.
Change-Id: I2076e4fc6719b6f06fd2796cbbc7548ec1876cb3
Reviewed-on: https://go-review.googlesource.com/6733 Reviewed-by: Rob Pike <r@golang.org> Reviewed-by: Ian Lance Taylor <iant@golang.org>