This is more consistent with the current naming scheme (i.e. IsCallable
for callable bit on map, IsConstructor for constructor bit on map, and
now IsUndetectable for undetectable bit on map).

Also simplify the fallthrough case for Object::Equals, because we don't
need to check for Null or Undefined or Undetectable, as both Null and
Undefined already have the undetectable bit set on their maps.

R=machenbach@chromium.org

Review URL: https://codereview.chromium.org/1756413003

Cr-Commit-Position: refs/heads/master@{#34458}