Under -Xsource:3, don't auto-eta-expand nilary methods to SAM types

[dwijnand]

A piece of code such as:

class C(s: java.io.InputStream)
class D(u: java.net.URL) extends C(u.openStream)

could be interpreted in two ways.

1. InputStream is a SAM of def read(): Int, and C(u.openStream) requires an eta-expansion from openStream method to () => Int.
2. u.openStream needs auto application ().

Under -Xsource:3 Scala 2.13.2 took the first interpretation, since #7660, and then errored with surprising:

       error: type mismatch;
        found   : java.io.InputStream
        required: Int

This PR fixes that by not eta-expanding nilary methods when a SAM type is expected.

Fixes scala/bug#12006; supersedes #9027

[lrytz]

very nice cleanup

[SethTisue]

@dwijnand I brought over Eugene's description from #9027; can you see if it needs any further adjustment/clarification?

[dwijnand]

Fixed.