[AssignmentTracking][Docs] Add new metadata to LangRef.rst and link to AssignmentTracking.md

[Orlando]

Summary

Related discourse thread: https://discourse.llvm.org/t/debuginfo-optimization-tests-and-automatic-check-generation

[jmorse]

Rather than store instructions, "instructions that store", seeing how you're marking some memory intrinsics with DIAssignID too?

Mega-nit: I'd suggest not using contractions in the super-formal LangRef document ("They're" -> "They are").

[Orlando]

Thanks @jmorse

+ "store instructions" -> "instructions that store"
+ "They're" -> "They are"

[jryans]

Seems reasonable to me, thanks for adding this! 🙂

[Orlando]

Thanks

llvm/docs/LangRef.rst

.. code-block:: text
     !0 = !DIExpression(DW_OP_deref)
     !1 = !DIExpression(DW_OP_plus_uconst, 3)
     !1 = !DIExpression(DW_OP_constu, 3, DW_OP_plus)
     !2 = !DIExpression(DW_OP_bit_piece, 3, 7)
     !3 = !DIExpression(DW_OP_deref, DW_OP_constu, 3, DW_OP_plus, DW_OP_LLVM_fragment, 3, 7)
     !4 = !DIExpression(DW_OP_constu, 2, DW_OP_swap, DW_OP_xderef)
     !5 = !DIExpression(DW_OP_constu, 42, DW_OP_stack_value)
 
+DIAssignID
+""""""""""""
+
+``DIAssignID`` nodes have no operands and are always distinct. They are used to
+link together `@llvm.dbg.assign` intrinsics (:ref:`debug
+intrinsics<dbg_intrinsics>`) and instructions that store in IR. See `Debug Info
+Assignment Tracking <AssignmentTracking.html>`_ for more info.
+
+.. code-block:: llvm
+
+    store i32 %a, ptr %a.addr, align 4, !DIAssignID !2
+    llvm.dbg.assign(metadata %a, metadata !1, metadata !DIExpression(), !2, metadata %a.addr, metadata !DIExpression()), !dbg !3
+
+    !2 = distinct !DIAssignID()
+
 DIArgList
 """"""""""""
 
 ``DIArgList`` nodes hold a list of constant or SSA value references. These are
 used in :ref:`debug intrinsics<dbg_intrinsics>` (currently only in
 ``llvm.dbg.value``) in combination with a ``DIExpression`` that uses the
 ``DW_OP_LLVM_arg`` operator. Because a DIArgList may refer to local values
 within a function, it must only be used as a function argument, must always be