Typecheck an overloaded label as a "head"

When implementing Quick Look I'd failed to remember that overloaded
labels, like #foo, should be treated as a "head", so that they can
be instantiated with Visible Type Application.

This patch fixes the problem by moving the typechecking for overloaded
labels from GHC.Tc.Gen.Expr to GHC.Tc.Gen.Head.

It's also done a bit more elegantly, rather than by constructing a new
HsExpr and re-invoking the typechecker.

This was all shown up by #19154, which is now fixed.