[MIR] Add MIR parsing for heap alloc site instruction markers

Summary:
This patch adds MIR parsing and printing for heap alloc markers, which were
added in D69136. They are printed as an operand similar to pre-/post-instr
symbols, with a heap-alloc-marker token and a metadata node.

Reviewers: rnk

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69864