go/types, types2: rename Environment to Context

Replace the name Environment with Context, as discussed in #47916. Along
the way, fix some stale or inaccurate comments.

The Environment type remains temporarily as an alias for Context, to
allow the x/tools Trybot to pass until dependency on types.Environment
can be removed.

Updates #47916

Change-Id: Iffd069ab0e8adebf4207c8f8891468a64d32b7cc
Reviewed-on: https://go-review.googlesource.com/c/go/+/353089
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>

src/cmd/compile/internal/noder/irgen.go

@@ -34,13 +34,13 @@ func checkFiles(noders []*noder) (posMap, *types2.Package, *types2.Info) {
 	}
 
 	// typechecking
-	env := types2.NewEnvironment()
+	env := types2.NewContext()
 	importer := gcimports{
 		env:      env,
 		packages: map[string]*types2.Package{"unsafe": types2.Unsafe},
 	}
 	conf := types2.Config{
-		Environment:           env,
+		Context:               env,
 		GoVersion:             base.Flag.Lang,
 		IgnoreLabels:          true, // parser already checked via syntax.CheckBranches mode
 		CompilerErrorMessages: true, // use error strings matching existing compiler errors