Wire up the flags needed to support module libraries built and used with
Swift. We need to pass `-bundle` to the linker when linking module
libraries on Darwin, and we need to pass `-export-dynamic` to the linker
when emitting an executable that exports symbols on Linux. This patch
wires up `CMAKE_SHARED_MODULE_CREATE_Swift_FLAGS` and
`CMAKE_SHARED_MODULE_LOADER_Swift_FLAG` on Darwin, and hooks up
`CMAKE_EXE_EXPORTS_Swift_FLAG` on Linux in order to support passing
things correctly.

We can't expose `CMAKE_EXE_LINKER_FLAGS` to Swift, as it contains flags
that the Swift compiler doesn't recognize, but the other
language-specific variables are safe to expose.