[lldb] Allow expect_expr without a running target

Summary:
If we don't have a current frame then we can still run many expressions
as long as we have an active target. With this patch `expect_expr` directly
calls the target's EvaluateExpression function when there is no current frame.

Reviewers: labath

Reviewed By: labath

Subscribers: JDevlieghere

Differential Revision: https://reviews.llvm.org/D77197