[ScriptInterpreter] Limit LLDB's globals to interactive mode.

Jim pointed out that the LLDB global variables should only be available
in interactive mode. When used from a command for example, their values
might be stale or not at all what the user expects. Therefore we want to
explicitly make these variables unavailable.

Differential revision: https://reviews.llvm.org/D67685

llvm-svn: 372192