Also add end-to-end tests for this finally, and fix a bug in
the SwiftReflectionTest library where we would give up on an
module completely if it did not have a field metadata section.
This is of course wrong if the module defines closures but
not nominal types.
Implement the ReflectionContext's implementation of:
swift_reflection_projectExistential.
First, we get the type info of the existential typeref - it should be a
record type info. If it's a class existential, it has trivial layout:
the first word is a pointer to the class instance. Otherwise, if the
value fits in the 3-word buffer of the existential container, it
trivially is also at the start of the container. Otherwise, the value is
off in a heap box somewhere, but the first word of the container is a
pointer to that box.
