1) Formalize "OnAccessor". A hack used to alias this to "OnFunc".
2) New aggregates: OnNominalType, OnGenericType, OnAbstractFunction.
3) Consistent and self-documented (albeit custom) style to ease code review/audits.
4) Removes a few cases of `OnAccessor` based on pull request #16031 feedback.
When calling a throwing function without 'try', let's suggest multiple
possibilities of note + fix-it for user to choose from.
Resolves: rdar://problem/33040113