Fix ctx implicits under case unapplySeq

[dwijnand]

A case class with a varargs has a unapplySeq extractor instead of
unapply. When we type an unapply, in typedUnapply, we first look for
unapply methods before unapplySeq methods. But when searching for
unapply, if a class method isn't found, then an extension method is
looked for, which causes context implicits to be cached. The bindings
from a pattern (such as from an unapply or unapplySeq extractor) are
added to the context in indexPattern. But Context's implicitCache
doesn't account for the scope changing.

I opted for giving the body its own scope context, rather than making
indexPattern reset the context implicits cache.

Fixes #21742