[llvm] [DirectX] Fixes for generation of DISubprogram (PR #194418)

[Harald van Dijk via llvm-commits]

hvdijk wrote:

> It'd probably be better to do this as several separate small PRs, with each explaining in the commit message why it's being done. A commit message that just lists a bunch of things the patch itself does isn't the most helpful.

These aren't a bunch of unrelated changes though, this is all part of "Emit DISubprogram in a way compatible with LLVM 3.7", I can see if I can word that clearer in the commit message / PR description. I could technically split this up but the bitcode parsing is fragile (see also #190611) and I don't think it's a good idea to make it so that we have to worry about the possibility of bitcode being used that was generated by LLVM snapshots that have some but not all of the fixes.

I have another pending change that affects how this PR works (without affecting what it does); since this isn't merged yet, to reduce churn I'll update this to do it that different way right away.

https://github.com/llvm/llvm-project/pull/194418