[lldb] Fix that the expression commands --top-level flag overwrites --allow-jit false

The `--allow-jit` flag allows the user to force the IR interpreter to run the
provided expression.

The `--top-level` flag parses and injects the code as if its in the top level
scope of a source file.

Both flags just change the ExecutionPolicy of the expression:
* `--allow-jit true` -> doesn't change anything (its the default)
* `--allow-jit false` -> ExecutionPolicyNever
* `--top-level` -> ExecutionPolicyTopLevel

Passing `--allow-jit false` and `--top-level` currently causes the `--top-level`
to silently overwrite the ExecutionPolicy value that was set by `--allow-jit
false`. There isn't any ExecutionPolicy value that says "top-level but only
interpret", so I would say we reject this combination of flags until someone
finds time to refactor top-level feature out of the ExecutionPolicy enum.

The SBExpressionOptions suffer from a similar symptom as `SetTopLevel` and
`SetAllowJIT` just silently disable each other. But those functions don't have
any error handling, so not a lot we can do about this in the meantime.

Reviewed By: labath, kastiglione

Differential Revision: https://reviews.llvm.org/D91780