[CMake] Install swiftinterface files alongside swiftmodules

This may not be their final destination, but at least this will
get them into built toolchains, which means it'll be easier to
do integration testing once all the other pieces are in place.

rdar://problem/43823937

Author: jrose-apple

cmake/modules/SwiftSource.cmake

@@ -334,7 +334,7 @@ function(_compile_swift_files
   endif()
 
   swift_install_in_component("${SWIFTFILE_INSTALL_IN_COMPONENT}"
-    FILES "${module_file}" "${module_doc_file}"
+    FILES "${module_file}" "${module_doc_file}" "${interface_file}"
     DESTINATION "lib${LLVM_LIBDIR_SUFFIX}/swift/${library_subdir}")
 
   set(line_directive_tool "${SWIFT_SOURCE_DIR}/utils/line-directive")