Need to hand-annotate this for now.

Until we get Clang support for "annotate all structure fields", we have to add this annotation by hand.
Add missing audit record.

If we want to prove this is missing, we can comment it out.
Warn about mixed simple, compound assignments.

It's not a good idea to try and mix instrumentation of simple assignments (x.foo = 42) with compound assignments (x.foo += 1 / x.foo++), as events can alias and confuse the runtime.

When the programmer specifies simple and compound assignments to the same ValueDecl, emit a warning diagnostic.