Push location information more tightly into the IR, providing space for every

operation and statement to have a location, and make it so a location is
required to be specified whenever you make one (though a null location is still
allowed).  This is to encourage compiler authors to propagate loc info
properly, allowing our failability story to work well.

This is still a WIP - it isn't clear if we want to continue abusing Attribute
for location information, or whether we should introduce a new class heirarchy
to do so.  This is good step along the way, and unblocks some of the tf/xla
work that builds upon it.

PiperOrigin-RevId: 210001406