[PATCH] Refactor VariantMatcher to use a tagged union.

[Samuel Benzaquen]

  We can't have a Matcher<T> as an out argument, because it doesn't have a default constructor.
  I could change the interface to return an llvm::Optional<Matcher<T>> instead, which doesn't require one.
  It would remove the has/get duality. The caller would always call get() and then check hasValue() in it.

http://llvm-reviews.chandlerc.com/D1446