Add an option to suppress the persistent result variable when running EvaluateExpression
from Python.  If you don't need to refer to the result in another expression, there's no
need to bloat the persistent variable table with them since you already have the result
SBValue to work with.

<rdar://problem/17963645>


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@215244 91177308-0d34-0410-b5e6-96231b3b80d8