[Assignment Tracking][6/*] Add trackAssignments function

The Assignment Tracking debug-info feature is outlined in this RFC:

https://discourse.llvm.org/t/
rfc-assignment-tracking-a-better-way-of-specifying-variable-locations-in-ir

Add trackAssignments which adds assignment tracking metadata to a function for
a specified set of variables. The intended callers are the inliner and the
front end - those calls will be added in separate patches.

I've added a pass called declare-to-assign (AssignmentTrackingPass) that
converts dbg.declare intrinsics to dbg.assigns using trackAssignments so that
the function can be easily tested (see
llvm/test/DebugInfo/Generic/track-assignments.ll). The pass could also be used
by front ends to easily test out enabling assignment tracking.

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D132225