Add DumpBinaryEscaped method to JSONGenerator, avoid extra copy

All uses of JSONGenerator in debugserver would create a JSON text
dump of the object collection, then copy that string into a
binary-escaped string, then send it up to the lldb side or
make a compressed version and send that.

This adds a DumpBinaryEscaped method to JSONGenerator which
does the gdb remote serial protocol binary escaping directly,
and removes the need to pass over the string and have an
additional copy in memory.

Differential Revision: https://reviews.llvm.org/D122882
rdar://91117456