[PrintAsObjC] Separate out module contents printing too (swiftlang#27184)

The top-level PrintAsObjC now just covers import printing and the
near-literal prologue/epilogue text, while the decl printing (and
import collecting) is in its own file. No functionality change.

Author: jrose-apple

lib/PrintAsObjC/CMakeLists.txt

@@ -1,5 +1,6 @@
 add_swift_host_library(swiftPrintAsObjC STATIC
   DeclAndTypePrinter.cpp
+  ModuleContentsWriter.cpp
   PrintAsObjC.cpp)
 target_link_libraries(swiftPrintAsObjC PRIVATE
   swiftAST