[cxx-interop] Fix crash with reverse interop in Embedded Swift
Embedded Swift has a minimal runtime, some type metadata is not
available. This patch works around a crash that tries to emit C++
briding to this non-existent Swift metadata. It is very likely that
there will be more fallout in reverse interop, but this patch should fix
the most glaring issue, crashing on an empty Embedded Swift project.
rdar://129030521