[PrintAsObjC] Pull the actual decl/type printer out to a separate file

I've tried several times to separate the type printing from the
decl-printing and found it tricky, but at least both can be separated
from the logic of walking the top-level decls in a module and
scheduling them appropriately. No functionality change.

Author: jrose-apple

lib/PrintAsObjC/CMakeLists.txt

@@ -1,4 +1,5 @@
 add_swift_host_library(swiftPrintAsObjC STATIC
+  DeclAndTypePrinter.cpp
   PrintAsObjC.cpp)
 target_link_libraries(swiftPrintAsObjC PRIVATE
   swiftAST