gdb/dwarf: add dwarf2_cu::find_die method

I added this small helper method in the series I'm writing, to make
finding a DIE by section offset a bit nicer than using the unordered_set
methods.  It doesn't have any dependencies, so I thought I would submit
it on its own.

Change-Id: If7313194ab09d9bd6d6a52c24eb6fd9a9c1b76e0
Approved-by: Kevin Buettner <kevinb@redhat.com>