[lldb/Reproducers] Support multiple GDB remotes

When running the test suite with always capture on, a handful of tests
are failing because they have multiple targets and therefore multiple
GDB remote connections. The current reproducer infrastructure is capable
of dealing with that.

This patch reworks the GDB remote provider to support multiple GDB
remote connections, similar to how the reproducers support shadowing
multiple command interpreter inputs. The provider now keeps a list of
packet recorders which deal with a single GDB remote connection. During
replay we rely on the order of creation to match the number of packets
to the GDB remote connection.

Differential revision: https://reviews.llvm.org/D71105