This replaces the vtable on AstNode with a NodeType tag. The visitors replace double dispatch with a single switch over the NodeType.

For now, visitors with subclasses still have virtual methods themselves. We should probably specialize them later as well.

The uint8_t NodeType allows us to better pack memory, saving 8-16 bytes on many AST nodes (with additional packing that I'll do in a follow-up CL)

BUG=

Review-Url: https://codereview.chromium.org/2142233003
Cr-Commit-Position: refs/heads/master@{#37788}