[lldb] [gdb-remote] Use Communication::WriteAll() over Write()

Replace the uses of Communication::Write() with WriteAll() to avoid
partial writes.  None of the call sites actually accounted for that
possibility and even if it is unlikely to actually happen, there doesn't
seem to be any real harm from using WriteAll() instead.

Ideally, we'd remove Write() from the public API.  However, that would
change the API of SBCommunication.  The alternative would be to alias it
to WriteAll().

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.llvm.org/D132395