This inserts a new bit set type Function, which is used to represent
JSFunctions, and uses that type in typed lowering to optimize calls
to use the CallFunction builtin directly. Also allows for better typing
of the typeof operator, which can infern "function" for JSFunctions
properly.

R=jarin@chromium.org

Review URL: https://codereview.chromium.org/1410343016

Cr-Commit-Position: refs/heads/master@{#31827}