Switch '/bin/bash' to '/bin/sh'

/bin/sh is more portable, and all systems with /bin/bash are expected to
have /bin/sh as well, even if only a link to bash.

Review: http://llvm-reviews.chandlerc.com/D1576


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@189879 91177308-0d34-0410-b5e6-96231b3b80d8