IR: Expose ModuleSlotTracker in Value::print()

IR: Expose ModuleSlotTracker in Value::print()

Allow callers of `Value::print()` and `Metadata::print()` to pass in a
`ModuleSlotTracker`.  This allows them to pay only once for calculating
module-level slots (such as Metadata).

This is related to PR23865, where there was a huge cost for
`MachineFunction::print()`.  Although I don't have a *particular* user
in mind for this new code, I have hit big slowdowns before when running
`opt -debug`, and I think this will be useful.  Going forward, if
someone hits a big slowdown with `print()` statements, they can create a
`ModuleSlotTracker` and send it through.  Similarly, adding support to
`Value::dump()` and `Metadata::dump()` should be trivial.

I added unit tests to be sure the `print()` functions actually behave
the same way with and without the slot tracker.

llvm-svn: 240867