[Reproducers] Capture and replay interpreter commands.

This patch adds the necessary logic to capture and replay commands
entered into the command interpreter. A DataRecorder shadows the input
and writes its data to a know file. During replay this file is used as
the command interpreter's input.

It's possible to the command interpreter more than once, with a
different input source. We support this scenario by using multiple
buffers. The synchronization for this takes place at the SB layer, where
we create a new recorder every time the debugger input is changed.
During replay we use the corresponding buffer as input.

Differential revision: https://reviews.llvm.org/D58564

llvm-svn: 355249