cmd/vet: make struct tag literal test work better with no go/types

Eliminate false positives when you can tell even without
type information that the literal does not need field tags.

Far too noisy otherwise.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/7797043
cmd/go: add go1.1 build tag, add -installsuffix flag

The new build tag "go1.1" will be satisfied by any Go 1.z release >= 1.1.
In general, the build tag "go1.x" will be satisfied by any Go 1.z release >= 1.x.
What happens when we reach Go 2 is yet to be decided.

The tags "go1" or "go1.0" are missing, because +build tags did not exist
before then, and also because the Go 1.0 releases do not recognize them.

The new -installsuffix flag gives access to the build context's InstallSuffix
(formerly named InstallTag, but not part of Go 1.0), for use in isolating
builds to custom directories. For example -race implies -installsuffix race,
and an AppEngine-specific build might use -tags appengine -installsuffix appengine.

Fixes issue 4116.
Fixes issue 4443.

R=golang-dev, bradfitz, r
CC=golang-dev
https://codereview.appspot.com/7794043