This is a requirement to replace AstNode vtable with a NodeType tag. In a second CL (https://codereview.chromium.org/2142233003/) I switch over that tag rather than double dispatch. With additional packing in a third CL there's roughly 8 byte reduction for almost all AstNode types, and for some even more.

BUG=

Review-Url: https://codereview.chromium.org/2126233002
Cr-Commit-Position: refs/heads/master@{#37770}