[lldb] Make expect_expr fall back to the dummy target if no target is selected

Summary:

Currently expect_expr will not run the expression if no target is selected. This
patch changes this behavior so that expect_expr will instead fall back to the
dummy target similar to what the `expression` command is doing. This way we
don't have to compile an empty executable to be able to use `expect_expr` (which
is a waste of resources for tests that just test generic type system features).

As a test I modernized the TestTypeOfDeclTypeExpr into a Python test +
expect_expr (as it relied on the dummy target fallback of the expression
command).

Reviewers: labath, JDevlieghere

Reviewed By: labath

Subscribers: abidh

Differential Revision: https://reviews.llvm.org/D83388