[Reproducer] Move GDB Remote Provider into Reproducer (NFC)

Originally the idea was for providers to be defined close to where they
are used. While this helped designing the providers in such a way that
they don't depend on each other, it also means that it's not possible to
access them from a central place. This proved to be a problem for some
providers and resulted in them living in the reproducer class.

The ProcessGDBRemote provider is the last remaining exception. This
patch makes things consistent and moves it into the reproducer like the
other providers.

llvm-svn: 371685