Sema: Fix a couple of problems with capture analysis and TopLevelCodeDecls
A TopLevelCodeDecl is a local context and any declarations inside
of one must be treated as captures. Furthermore, all the
TopLevelCodeDecl children of a source file are peers, and can
see each other's bindings, so don't perform an exact match on the
DeclContext.
Part of <rdar://problem/23051362> / <https://bugs.swift.org/browse/SR-3528>.