swift234

Rework our handling of "external" definitions created by the Clang importer.

Keep track of external definitions as they are created by broadcasting
them through a mutation listener interface. At name binding time, we
just cache them. When a type checker is alive, it immediately performs
any additional operations necessary on those types (e.g., declaring
implicit constructors).

This also eliminates some O(N^2) behavior in the type checker as well,
because we don't have to walk through all of the module imports to
find the external definitions. We just keep a single list in the
ASTContext along with our place in the list.

Fixes <rdar://problem/13769497>.


Swift SVN r5032