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

The changes between (equivalent, and reviewed) go/types/stdlib_test.go
and stdlib_test.go can be seen by comparing patchset 1 and 2. The actual
changes are removing the "// UNREVIEWED" marker, using the os package
instead of ioutil, and some comment adjustments. Also, bug251.go passes
because of recent changes.

The primary difference is in the firstComment function which
doesn't have access to a scanner and instead uses the syntax
package's CommentsDu function.

Change-Id: I946ffadc97e87c692f76f369a1b16cceee528477
Reviewed-on: https://go-review.googlesource.com/c/go/+/304130


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>