Variable: Fix usage of uninitialised value

Summary:
Variable::GetValuesForVariableExpressionPath was passing an
uninitialised value for the final_task_on_target argument. On my
compiler/optimization level combo, the final_task_on_target happened to
contain "dereference" in some circumstances, which produced hilarious
results. The same is true for other arguments to the
GetValueForExpressionPath call.

The correct behavior here seems to be to just omit the arguments
altogether and let the default behavior take place.

Reviewers: jingham

Subscribers: mehdi_amini, lldb-commits

Differential Revision: https://reviews.llvm.org/D40557

llvm-svn: 320021