[SILGen] Implement SILGenTopLevel

Implement SILGenTopLevel, a class that walks a file
run in script mode to generate all toplevel code
at once.

Author: Zak Kent

lib/SILGen/CMakeLists.txt

@@ -35,6 +35,7 @@ add_swift_host_library(swiftSILGen STATIC
   SILGenRequests.cpp
   SILGenStmt.cpp
   SILGenThunk.cpp
+  SILGenTopLevel.cpp
   SILGenType.cpp)
 target_link_libraries(swiftSILGen PRIVATE
   swiftSerialization