Added support for setting null-conditional access receiver state.

* features/NullableReferenceTypes:
  `is` operator informs nullability analysis (#28686)
  Make module-level NonNullTypes explicit in tests (#28769)
  Lazily evaluate NonNullTypes (#28736)
  Calculate TypeSymbolWithAnnotations.IsNullable lazily (#28687)

Merge 'upstream/dev15.8.x' into 'features/NullableReferenceTypes'

Turn on loc check for dev15.8.x

Turn on the `windows_loc_status` check for the dev15.8.x and
dev15.8.x-vs-deps branches for CI builds. For PR builds it will continue
to show only when requested.

It is expected that it will fail in these branches until we have our
final loc passes.

Handle null in AssemblyVersion attribute

Remove unused Maddog code

Erase nullability for unconstrained type parameters in constraints from unannotated assemblies (#28282)

Maddog is no longer used here hence there is no need to generate these
files anymore. Note: their contents were incorrect anyways at this point
which is what lead us to look into whether or not they were being used
anymore.

* Track enclosing symbol in data flow analysis, which might be a field or property.
Code used to assume it was (and typed it as) a method.
Fixes #19845
Fixes #27969

Provide a spellchecking suggestion when someone mispells a constructor.