<rdar://problem/13831149>

Recursive commands invocations are not currently supported by our CommandInterpreter
CommandScriptImport can actually be made to invoke itself recursively, so we need to work around that by clearing the m_exe_ctx
This is a short-term workaround, a more interesting solution would be to actually make sure recursive command invocations work properly

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@181537 91177308-0d34-0410-b5e6-96231b3b80d8