gdb: add another overload of startswith

We already have one overload of the startswith function that takes a
std::string_view for both arguments.  A later patch in this series is
going to be improved by having an overload that takes one argument as
a std::string_view and the other argument as a plain 'char *'.

This commit adds the new overload, but doesn't make use of it (yet).
There should be no user visible changes after this commit.