cmd/fix: add buildtag fix

cmd/fix: add buildtag fix

Now that Go 1.17 is out and Go 1.15 is unsupported,
removing // +build lines can be done safely: in the worst case,
if code is compiled using Go 1.16 the toolchain will detect
the presence of a //go:build without // +build and fail the build.
(It will not silently choose the wrong files.)

Note that +build lines will continue to work in Go sources forever.
This just provides a mechanism for users who are done with
Go 1.16 to remove them easily, by running "go fix".

Also update for new generics AST.

For #41184.
Fixes #48978.

Change-Id: I11a432c319e5abd05ad68dda9ccd7a7fdcc8bbb8
Reviewed-on: https://go-review.googlesource.com/c/go/+/240611
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>