go/analysis/passes/stdversion: test *.go < go.mod version

This CL adds two tests for when the file's Go version
(set by a //go:build constraint) is lower than the module's
Go version.

With a go.mod version < go1.21, the start of the "extended forward
compatibility" regime, stdversion is silent. But with go1.21+,
the build constraint in the file overrides the Go module version,
even when lower.

Fixes golang/go#67123

Change-Id: Iba37ff2d2c6d0e42a5bd0fe80bbd8dc6b1a25ac9
Reviewed-on: https://go-review.googlesource.com/c/tools/+/582936
Reviewed-by: Tim King <taking@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

go/analysis/passes/stdversion/stdversion_test.go

@@ -23,5 +23,6 @@ func Test(t *testing.T) {
 	analysistest.Run(t, dir, stdversion.Analyzer,
 		"example.com/a",
 		"example.com/sub",
+		"example.com/sub20",
 		"example.com/old")
 }

go/analysis/passes/stdversion/testdata/test.txtar

@@ -13,6 +13,7 @@ go 1.21
 
 use .
 use sub
+use sub20
 use old
 
 -- go.mod --
@@ -106,3 +107,42 @@ package old
 import "go/types"
 
 var _ types.Alias // no diagnostic: go.mod is too old for us to care
+
+-- sub/oldtagged.go --
+// The file Go version (1.16) overrides the go.mod Go version (1.21),
+// even when this means a downgrade (#67123).
+// (stdversion is silent for go.mod versions before 1.21:
+// before the forward compatibility regime, the meaning
+// of the go.mod version was not clearly defined.)
+
+//go:build go1.16
+
+package sub
+
+import "bytes"
+import "go/types"
+
+var _ = bytes.Clone // want `bytes.Clone requires go1.20 or later \(file is go1.16\)`
+var _ = types.Alias // want `types.Alias requires go1.22 or later \(file is go1.16\)`
+
+-- sub20/go.mod --
+module example.com/sub20
+
+go 1.20
+
+-- sub20/oldtagged.go --
+// Same test again, but with a go1.20 mod,
+// before the forward compatibility regime:
+// The file's build tag effects selection, but
+// not language semantics, so stdversion is silent.
+
+//go:build go1.16
+
+package sub
+
+import "bytes"
+import "go/types"
+
+var _ = bytes.Clone
+var _ = types.Alias
+