Fix completions in return when in function with contextual 'this'

[rbuckton]

This change forces the containing function to be type-checked when requesting completions so that contextual this parameters in the containing function are already checked (and cached). Prior to this change, pulling on a type while requesting completions could result in a circularity that doesn't exist at compile time.

Fixes #39946

[weswigham]

Precaching the type in the services layer works, although I wonder if maybe instead we should be doing it during the normal checking process, rather than relying on tree ordering ensuring things are in the cache as needed?

[rbuckton]

getCompletionData calls program.getTypeChecker(), so I'm not sure if this is a side-effect of a non-diagnostics-producing checker, or some other reason why the this type isn't already cached.

[rbuckton]

I'll go with this solution for now, but will revisit if necessary.