Ninja,Swift: reuse `SWIFT_MODULE_NAME` for `SWIFT_MODULE`

Rather than compute the value of SWIFT_MODULE, use the computed value of SWIFT_MODULE_NAME. This is helpful strictly for Windows where you have multiple configuration variants with differing ABIs that need must co-exist. In particular, this re-use permits the use of CMAKE_DEBUG_POSTFIX to have the import library and the DLL be named with the trailing suffix but have the same import name for the swift module and interface.