Zend fatals if the type is not string, where hphp would
coerce to string. In RepoAuthoritative mode, this meant that
scalar expressions would be converted to strings at bytecode
emission time, bypassing the fatal (or, in most cases, resulting
in a different fatal).
This change is mostly for FB internal organizational reasons.
Building is not effected beyond the fact that the target now
lands in hphp/hhvm/hhvm rather than src/hhvm/hhvm.