go/build: add go1.12 release tag

Change-Id: I82e3f9140e5d14f02beef64c474a3ae88fe256e1
Reviewed-on: https://go-review.googlesource.com/c/147219
Run-TryBot: Alan Donovan <adonovan@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/go/build/build.go

@@ -298,7 +298,7 @@ func defaultContext() Context {
 	// (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.
 	// NOTE: The last element in ReleaseTags should be the current release.
-	const version = 11 // go1.11
+	const version = 12 // go1.12
 	for i := 1; i <= version; i++ {
 		c.ReleaseTags = append(c.ReleaseTags, "go1."+strconv.Itoa(i))
 	}

src/go/build/doc.go

@@ -108,6 +108,7 @@
 //	- "go1.9", from Go version 1.9 onward
 //	- "go1.10", from Go version 1.10 onward
 //	- "go1.11", from Go version 1.11 onward
+//	- "go1.12", from Go version 1.12 onward
 //	- any additional words listed in ctxt.BuildTags
 //
 // There are no build tags for beta or minor releases. Programs that need the