Discourage default arg for extension receiver (scala#22492)

Fixes scala#12460 

I shied away from making it an error, as that is a language change that
violates the rule that extension methods are ordinary methods. There are
other restrictions, but an extension always allows explicit invocation
`m()(x)` that could leverage a default.

The caret is wrong on the second test case (~todo~). Edit: span of
default getter was union of first parameter and the param RHS, so that
the synthetic position of the getter symbol was a point at the first
parameter. Now the getter tree gets the span of the RHS. (That span is
non-synthetic, but the definition is still synthetic. The name pos of a
synthetic is the point.)
[Cherry-picked afb4806][modified]

Author: tgodzik