cmd/vet: Use function signature to find format string index.

cmd/vet: Use function signature to find format string index.

cmd/vet's printf checker currently uses a hardcoded map of function
names to expected positions of format strings. We can be a bit more
precise than this by looking up the signature of the function, which
helps when libraries implement functions like Errorf or Logf with
extra arguments like log levels or error codes.

Specifically, the format string param is assumed to be the last string
parameter of the called function.

Fixes #12294.

Change-Id: Icf10ebb819bba91fa1c4109301417042901e34c7
Reviewed-on: https://go-review.googlesource.com/20163
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>