A pretty severe bug in the recognition of tail ...

A pretty severe bug in the recognition of tail call elimination.
It turns out that Tailcalls will perform "partial elimination" in
situations such as:

  @annotation.tailrec final def f(x: Int): Int = f(f(x))

The outer call to f1 becomes a jump, but the inner call remains as
it was. I implemented @tailrec under the impression that if the
optimization had taken place, it had gone all the way. So this is now
fixed with a direct examination of the rewritten tree.

While I was in there I threw in some improved error reporting: the error
positioning is now on the call which is not in tail position rather than
the method declaration.

Closes #4135, no review.

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