go/types: add Checker.walkDecl to simplify checking declarations
Handling ast.GenDecls while typechecking is repetitive, and a source of
diffs compared to typechecking using the cmd/compile/internal/syntax
package, which unpacks declaration groups into individual declarations.
Refactor to extract the logic for walking declarations. This introduces
a new AST abstraction: types.decl, which comes at some minor performance
cost. However, if we are to fully abstract the AST we will be paying
this cost anyway, and benchmarking suggests that the cost is negligible.
Change-Id: If73c30c3d08053ccf7bf21ef886f0452fdbf142e
Reviewed-on: https://go-review.googlesource.com/c/go/+/256298
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Robert Findley <rfindley@google.com>