Reapply: [Assignment Tracking][7/*] Add assignment tracking functionality to clang

Reverted in 98fa9549.

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

This patch plumbs the AssignmentTrackingPass (AKA declare-to-assign), added in
the previous patch in this set, into the optimisation pipeline from
clang. clang/test/CodeGen/assignment-tracking/assignment-tracking.cpp is the
main test for this patch.

Note: while clang (with the help of the declare-to-assign pass) can now emit
Assignment Tracking metadata, the llvm middle and back ends don't yet
understand it.

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D132226