Check for unsupported use of `synchronized` in async blocks

[retronym]

I think we should treat it as a by-name method for the purposes of our analysis / ANF transform.

scala> ObjectTpe.decl(TermName("synchronized")).defString
res2: String = final def synchronized[T0](x$1: T0): T0

[SethTisue]

@retronym should this be closed now that scala/scala#9082 has been merged?

[retronym]

Yep, thanks