Examine ExecutableType.getReceiverType behavior when source receiver parameter is absent

The receiver parameter language structure was added in Java SE 8 as part of the type annotations effort. Receiver parameters provide a syntactic structure to associate annotations, when need, with a method's receiver or a nested constructor's enclosing class.

The javax.lang.model API was augmented to include methods to return the receiver *type* (JDK-8007803). Conceptually, a receiver type exists whether or not the syntax of a receiver parameter is present.

However, javac's implementation of the language model only exposes non-trivial receiver types if the receiver parameter syntax was present in the sources.