How about this version?

Here I use libiberty/hashtab.c to determine function name ambiguity.

It turns out the bug exists for constructors too.  I.e.
l->function_name for "Derived" in the gdb.cp/mb-ctor testcase is
"Derived" for both locations.  I wonder if for this particular
situation l->function_name should record an "enhanced" name to
distinguish them.