* gdb.c++/derivation.exp, gdb.c++/overload.exp,

* gdb.c++/derivation.exp, gdb.c++/overload.exp,
gdb.c++/userdef.cc: Place comments on the lines to which the
marker function might return.
* gdb.c++/derivation.exp, gdb.c++/overload.exp,
gdb.c++/userdef.exp: Look for those comments to check that we've
returned to the right place, instead of checking line numbers.