This gets rid of the (litstr) StringData and StackStringData
constructors, but keeps String(litstr). Also rename all
the instances of AttachLiteral to CopyString, since they now
mean the same thing.
change callers of determine_charset to check for nulls, instead of calling html_supported_charset (to pre-validate charset name) and then calling this (which has to run through a list of names anyway).
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.