Add script to add HH_FIXME

Summary: Simple script to take the output of `hh_client --json` and add
a FIXME for every single one.

Reviewed By: @dlreeves

Differential Revision: D1614381
Remove CodeGenerator::m_slocs & m_dlocs

Summary: They're just copied from m_state.locs, so plumb that through
to the handful of callsites that need it, and plumb inst through
to srcLoc() and dstLoc().

Reviewed By: @jdelong

Differential Revision: D1626986