Add some kind of `@PolyNull`-like support for `Optional.orElseGet`?

[msridhar]

The JSpecify JDK model for Optional.orElseGet is (here):

    // We would use an annotation like @PolyNull here if JSpecify offered one.
    public @Nullable T orElseGet(Supplier<? extends @Nullable T> supplier) {

When working on #1613 I noticed that modeling orElseGet as returning @Nullable introduces many false positives, for cases where the provided supplier cannot return null. I think we probably need to find a way to model this one precisely in order to not cause too many FPs in practice.

[lazaroclapp]

Extending @Contract (or whatever the library model equivalent is) to allow referring to type bounds within arguments and return values, maybe?

[msridhar]

Yeah, this is beyond the expressivity of @Contract and I'm hesitant to add NullAway's own interpretation of contract strings. So probably something special in library models is the right solution for now, letting you model that if a type argument of a parameter type is @NonNull, so is the return type.