Proper nullable support for interpolated string handler constructors and conversions

[333fred]

Currently, we do a very basic form of nullable analysis for interpolated string handler conversions. We need to figure out how it will interact with method reinference and property propagate state from the constructor out to other components.

Relates to test plan #51499

[333fred]

#55701 (comment) is very related, and likely needs to be addressed at the same time to make sure we're not duplicating code across different flow passes.