Hang closure capture descriptors from their heap metadata

Now we can discern the types of values in heap boxes at runtime!
Closure reference captures are a common way of creating reference
cycles, so this provides some basic infrastructure for detecting those
someday.

A closure capture descriptor has the following:

- The number of captures.
- The number of sources of metadata reachable from the closure.
  This is important for substituting generics at runtime since we
  can't know precisely what will get captured until we observe a
  closure.
- The number of types in the NecessaryBindings structure.
  This is a holding tank in a closure for sources of metadata that
  can't be gotten from the captured values themselves.
- The metadata source map, a list of pairs, for each
  source of metadata for every generic argument needed to perform
  substitution at runtime.
  Key: The typeref for the generic parameter visible from the closure
  in the Swift source.
  Value: The metadata source, which describes how to crawl the heap from
  the closure to get to the metadata for that generic argument.
- A list of typerefs for the captured values themselves.

Follow-up: IRGen tests for various capture scenarios, which will include
MetadataSource encoding tests.

rdar://problem/24989531

include/swift/Reflection/MetadataSourceBuilder.h

@@ -62,11 +62,6 @@ public:
   createParent(const MetadataSource *Child) {
     return ParentMetadataSource::create(*this, Child);
   }
-
-  const ImpossibleMetadataSource *
-  createImpossible() {
-    return ImpossibleMetadataSource::get();
-  }
 };
 
 } // end namespace reflection