icmd/vet: improved checking for variadic Println-like functions
- Automatically determine the first argument to check.
- Skip checking matching non-variadic functions.
- Skip checking matching functions accepting non-interface{}
variadic arguments.
- Removed fragile 'magic' code for special cases such as math.Log
and error interface.
Fixes #15067
Fixes #15099
Change-Id: Ib313557f18b12b36daa493f4b02c598b9503b55b
Reviewed-on: https://go-review.googlesource.com/21513
Run-TryBot: Rob Pike <r@golang.org> Reviewed-by: Rob Pike <r@golang.org>