[ClangImporter] move swiftify to SwiftifyDecl.cpp (NFC)

This code doesn't interact that much with the rest of ClangImporter, and
will continue to grow. Keeping it in a separate file makes it easier to
navigate.

Author: hnrklssn

lib/ClangImporter/CMakeLists.txt

@@ -24,6 +24,7 @@ add_swift_host_library(swiftClangImporter STATIC
   ImportName.cpp
   ImportType.cpp
   Serializability.cpp
+  SwiftifyDecl.cpp
   SwiftLookupTable.cpp
 )
 target_link_libraries(swiftClangImporter PRIVATE