Added a new AST transformation pass called the

"Playground Transform."  This is an
instrumentation pass that adds calls to a
function called playground_log at locations of
interest.  Roughly speaking, these locations are

- Initialization of variables
- Modification of variables
- Expressions returning values
- Application of mutating methods on objects

The playground transform currently only finds
modifications of variables, but the intent is to
make all of these cases work.

It is enabled by a frontend option, and can
also be invoked by calling

swift::performPlaygroundTransform(SF)

which is the way LLDB, its main client, will
use it.

The frontend option is intended for testing,
and indeed I will add tests for this
transformation in the coming week as I bring
more functionality online.


Swift SVN r14801

lib/Frontend/Frontend.cpp

@@ -308,6 +308,9 @@ void CompilerInstance::performParse() {
                             CurTUElem);
       CurTUElem = MainFile.Decls.size();
     } while (!Done);
+
+    if (Invocation.getFrontendOptions().Playground)
+      performPlaygroundTransform(MainFile);
   }
 
   if (!Invocation.getParseOnly()) {