Feature Request: add TypeChecker API to query if identifier is definitely assigned

[ajafff]

Currently it's not possible to query CFA. I need to find out if an identifier is definitely assigned. This logic is part of checkIdentifier and is not accessible outside of that function.

My use case is the TSLint rule no-unnecessary-type-assertion where we need to identify false positives. The current implementation compares the type before and after the non-null-assertion. That doesn't work well for identifiers that are not definitely assigned, because the assertion suppresses TS2454 and can therefore not be removed.