Rework the integrated REPL to use separate modules for every line

...like LLDB does, instead of parsing into a single SourceFile.
This does break some functionality:

- no more :dump_ast
- no redeclaration checking, but no shadowing either---redeclarations
  just become ambiguous
- pretty much requires EnableAccessControl to be off, since we don't
  walk decls to promote them to 'public'

...but it allows us to remove a bit of longstanding support for
type-checking / SILGen-ing / IRGen-ing only part of a SourceFile that
was only used by the integrated REPL.

...which, need I remind everyone, is still /deprecated/...but sometimes
convenient. So most of it still works.

lib/AST/Module.cpp

@@ -1305,6 +1305,12 @@ SourceFile::collectLinkLibraries(ModuleDecl::LinkLibraryCallback callback) const
     if (next->getName() == getParentModule()->getName())
       return true;
 
+    // Hack: Assume other REPL files already have their libraries linked.
+    if (!next->getFiles().empty())
+      if (auto *nextSource = dyn_cast<SourceFile>(next->getFiles().front()))
+        if (nextSource->Kind == SourceFileKind::REPL)
+          return true;
+
     next->collectLinkLibraries(callback);
     return true;
   });