Add autoload support for .hh files (HHVM)

HHVM is adding support for an alternative extension for files using
HHVM-specific features. Support them in the Class Map and PSR4
autoloaders.

Trivial example: https://github.com/fredemmott/hh_extension_toolset
Only look for .hh files at runtime if using HHVM

The ClassMap generator will look regardless, but that's not a runtime
cost, so seems desirable (in case composer install/update needs to be
called with php5 for some resaon)
Merge remote-tracking branch 'fredemmott/hh-support'
CS fixes, refs #2823