Fix #6946: Fix generic signature for synthetic function types

[odersky]

Generic signature generation did not take synthetic functions into account.

I lack the techniques how to test this. Maybe somebody else can add a test?

@smarter If the fix works, do we need then a new reference compiler before we can use IFTs in Reporter?

[smarter]

I lack the techniques how to test this. Maybe somebody else can add a test?

The test in b129009 should be easy to extend.

If the fix works, do we need then a new reference compiler before we can use IFTs in Reporter?

Yes, I think so.