When invoking Terminal, don't assume the default shell

Summary:
If a user has their shell set to a non-POSIX conferment shell the TestTerminal.py tests fail because the shell blurb constructed here may not work in their shell.

In my specific case fish-shell (The Friendly Interactive Shell - http://fishshell.com) does not support $?, it instead uses $status (because it is friendly).

This patch removes the assumption of your default shell by running the constructed bash command via "/bin/bash -c ...". This should be safer for users mutating their shell environment.

Reviewers: tfiala

Subscribers: joerg, lldb-commits

Differential Revision: https://reviews.llvm.org/D25750

llvm-svn: 284552