Move `Literal` flag detection into recurse phase (#5768)
## Summary
The AST pass is broken up into three phases: pre-visit (which includes
analysis), recurse (visit all members), and post-visit (clean-up). We're
not supposed to edit semantic model flags in the pre-visit phase, but it
looks like we were for literal detection. This didn't matter in
practice, but I'm looking into some AST refactors for which this _does_
cause issues.
No behavior changes expected.
## Test Plan
Good test coverage on these.
