Fix a hole in access control checking for var/let (#21168)

Previously if a declaration had both a Type and a TypeRepr available,
we would only check the access of the TypeRepr. However, this is
incomplete when the type is partially inferred, as in

  public var inferredGenericParameters: Optional = PrivateStruct()

The new algorithm is to the Type first, then:
- if the Type is okay, move on to check the TypeRepr
- if the Type is not okay and we're in pre-Swift-5 mode, check the
  TypeRepr, and if /that's/ okay downgrade the whole thing to a
  warning.

Unfortunately, we can't /just/ check the Type in the "good" case,
because we don't always properly preserve sugar when going from a
TypeRepr that represents a typealias to the corresponding Type, and we
want to be able to fix those cases in the future. So we have to check
both.

rdar://problem/46596085
(cherry picked from commit a5947fe)

Author: jrose-apple