The SILGen testsuite consists of valid Swift code covering most language
features. We use these tests to verify that no unknown nodes are in the
file's libSyntax tree. That way we will (hopefully) catch any future
changes or additions to the language which are not implemented in
libSyntax.
When lowering closures, we avoid capturing the enclosing generic context when possible. However, the generic context may still be necessary to canonicalize types mentioned inside the closure, such as when an associated type is referred to that is same-typed to a concrete type. Fixes rdar://problem/30254048.
