cmd/compile/internal/types2: review of api.go

The changes between (equivalent, and reviewed) go/types/api.go
and api.go can be seen by comparing patchset 1 and 2. The actual
changes are removing the "// UNREVIEWED" marker.

The primary differences to go/types/api.go are:
- use of syntax instead of go/ast package
- use of simpler Error type (for now)
- additional exported Config flags
- different handling of nil values (we can't use Typ[UntypedNil]
  to represent an untyped nil because types2 gives such nil values
  context-dependent types)

Change-Id: I7d46b29d460c656d7a36fe70108a370383266373
Reviewed-on: https://go-review.googlesource.com/c/go/+/304050
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/types2/api.go

@@ -1,4 +1,3 @@
-// UNREVIEWED
 // Copyright 2012 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
@@ -23,8 +22,6 @@
 // and checks for compliance with the language specification.
 // Use Info.Types[expr].Type for the results of type inference.
 //
-// For a tutorial, see https://golang.org/s/types-tutorial.
-//
 package types2
 
 import (