Remember to clear the cached data for the OptionValueFileSpec in SetValueFromCString.

<rdar://problem/16179718> target.expr-prefix contents appear to be cached for an entire session


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@205636 91177308-0d34-0410-b5e6-96231b3b80d8