[lldb] Enable use of dummy target from dwim-print

Allow `dwim-print` to evaluate expressions using the dummy target if no real
target exists.

This adds some parity to `expression`. With this, both of the following work:

```
lldb -o 'expr 1+2'
lldb -o 'dwim-print 1+2'
```

Differential Revision: https://reviews.llvm.org/D138960