Handle the options and parameters separator in every MI command

Summary:
As per the following link, the "--" separator can appear between the options
and parameters of any MI command. Previously this separator was only
handled by the `-data-disassemble` MI command. I have moved the relevant
code into `CMICmdBase` so that any MI command can handle the
aforementioned separator.

https://sourceware.org/gdb/onlinedocs/gdb/GDB_002fMI-Input-Syntax.html#GDB_002fMI-Input-Syntax

Reviewers: ki.stfu

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D14197

llvm-svn: 251793