This adds a method on the `PublicsStream` to look up symbols using their
address (segment + offset).
It's largely a reimplementation of
[`NearestSym`](805655a28b/PDB/dbi/gsi.cpp (L1492-L1581))
from the reference. However, we don't return the nearest symbol, but the
exact symbol.
Still, in case of ICF, we return the symbol that's first in the address
map. Users can then use the returned offset to read the next records to
check if multiple symbols overlap, if desired.
From #149701.
c82d09c96a