[llvm] [Debug] Add support for multi-level line support (PR #211390)

[Alexander Yermolovich via llvm-commits]

ayermolo wrote:

Re-formatted this PR into 3 commits that track PRs I'll create for review.
I thought of splitting MIR part, but it's just a small change to parser + test.
Kept it as part of first commit so round trip for mir works out of the gate.

First one:
https://github.com/llvm/llvm-project/pull/215666

https://github.com/llvm/llvm-project/pull/211390