Take exhaustive match into account when checking for field initialization

[SeanTAllen]

Previously, exhaustive matches were handled after we verified that an object's fields
were initialized. This lead to the code such as the following not passing compiler
checks:

primitive Foo
  fun apply() : (U32 | None) => 1

type Bar is (U32 | None)

actor Main
  let bar : Bar

  new create(env : Env) =>
      match Foo()
      | let result : U32 =>
          bar = result
      | None =>
          bar = 0
      end

This commit moves field initialization checking to the verify pass so that it
comes after the expression pass when exhaustiveness checking is done. This also
moves code to patchup matches from the refer pass to a new completeness pass
that comes after the expression pass. The new completeness_match code is
identical to refer_match code except that an else clause with a value of
TK_NONE no longer means a clause exists. In the exhaustiveness checks in the
expression pass, any non-exhaustive else clause that was a TK_NONE has since
been replaced with a else None so, by the time we get to completeness,
TK_NONE literally means "there's no else clause".

Closes #3615

[SeanTAllen]

Windows failure can be ignored.