Properly report versions in PHP7 mode
Summary:
This ended up being somewhat more annoying than you would hope, due to a couple of places that assumed the version was a compile-time constant. Also, wiring the switch into mangleUnitMd5 is weird, but in repo mode, `PHP_VERSION_MAJOR` can get inlined, so it does actually affect bytecode emission :\
Fixes https://github.com/facebook/hhvm/issues/6636
Reviewed By: paulbiss
Differential Revision:
D2732232
fb-gh-sync-id:
a54e2216d12fd875df698aa35c06669f1ea4d435