awakens default getter synthesis from the untyper nightmare

Our happy little macro paradise is regularly invaded by resetAllAttrs,
the bane of all macros and typers. It’s so ruthless and devastating that
we’ve been long scheming to hack something really cool and to one day
defeat it.

Today we make the first step towards the happy future. Today we overthrow
the UnTyper, resetAllAttrs’s elder brother that rules in the kingdoms of
GetterLand and CaseClassia, and banish him from the land of getters.
In the name of what’s good and meta, let’s band together and completely
drive him away in a subsequent pull request!

To put it in a nutshell, instead of using untyper on a DefDef to do
default getter synthesis, the commit duplicates the DefDef and does
resetLocalAttrs on it.

As default getter synthesis proceeds with figuring out type and value
parameters for the getter, then its tpt and finally its rhs, the commit
destructures the duplicated DefDef and then assembles the default getter
from the destructured parts instead of doing copyUntyped/copyUntypedInvariant
on the original DefDef.

I would say the test coverage is pretty good, as I had to figure out
3 or 4 test failures before I got to the stage when everything worked.
Iirc it tests pretty exotic stuff like polymorphic default parameters
in both second and third parameter lists, so it looks like we're pretty
good in this department.
untyper is no more

Unlike with default getters, removing untyper from case class synthesis
was trivial. Just resetLocalAttrs on a duplicate of the provided class def,
and that’s it.

resetAllAttrs, you’re next. We’ll get to you! Eventually...
Merge pull request #3305 from xeno-by/topic/copy-untyped

awakens default getter synthesis from the untyper nightmare