go/build: add go1.11 build tag

As usual, adding go1.11 early in the cycle so that we can start
regression testing of the master toolchain.

Change-Id: Ie96eca7223722d60d7acc6b3b996b76740c36419
Reviewed-on: https://go-review.googlesource.com/93775Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/go/build/build.go

@@ -292,7 +292,7 @@ func defaultContext() Context {
 	// say "+build go1.x", and code that should only be built before Go 1.x
 	// (perhaps it is the stub to use in that case) should say "+build !go1.x".
 	// NOTE: If you add to this list, also update the doc comment in doc.go.
-	const version = 10 // go1.10
+	const version = 11 // go1.11
 	for i := 1; i <= version; i++ {
 		c.ReleaseTags = append(c.ReleaseTags, "go1."+strconv.Itoa(i))
 	}

src/go/build/doc.go

@@ -107,6 +107,7 @@
 //	- "go1.8", from Go version 1.8 onward
 //	- "go1.9", from Go version 1.9 onward
 //	- "go1.10", from Go version 1.10 onward
+//	- "go1.11", from Go version 1.11 onward
 //	- any additional words listed in ctxt.BuildTags
 //
 // If a file's name, after stripping the extension and a possible _test suffix,