go/ast/astutil: handle documentation of imports in DeleteNamedImport.

If the parens of an ast.GenDecl are dropped, move the documentation
of the ImportSpec above the import statement, otherwise the the
code is invalid.

Fixes golang/go#15432.

Change-Id: I715750b8f528380b96a6bc8b5f216043937976c2
Reviewed-on: https://go-review.googlesource.com/22415
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
2 files changed