Allow calling closures inline

Summary: Add support for invokeing closures inline to the parser:
`(function () { echo "foo"; })()`

Fixes #3890

Reviewed By: @JoelMarcey

Differential Revision: D1662663

Signature: t1:1662663:1415249841:ce564a5ec1a2324dbca6c0e47db96ed85076c89e