spanned HIR node enums should be named FooKind rather than being named Foo_ and glob-exported

[zackmdavis]

@petrochenkov writes:

The convention enum Something_ { SomethingVariant } is "deprecated" and exists in HIR only because not all HIR structures were updated to the "modern" form enum SomethingKind { Variant } like it was done in AST.

New structures should not use it, so it'd be better to rename Visibility_ to VisibilityKind.

The referenced changes to AST were mostly in February 2016's #31487 (justification). The following HIR nodes are still using the underscore-suffix convention, and should probably be changed to the Kind style for consistency:

• BinOp_
• Stmt_
• Decl_
• Variant_

[varkor]

Presumably this applies to Expr_ too?

[zackmdavis]

Ah, yes: I was searching for Spanned, but the same rationale would apply to Expr_, Ty_, and ForeignItem_.

[oli-obk]

Since any of these changes will break clippy I would be delighted if a PR doing them would also update clippy with the relevant changes. Not a blocker though!