Clean up uses of the unstable dwarf_version option

[wesleywiser]

• Consolidate calculation of the effective value.
• Check the target DebuginfoKind instead of using is_like_msvc.
• Add the tracking issue to the unstable book page for this feature.

cc #103057

[rustbot]

r? @chenyukang

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[lqd]

Sweet, r? lqd and r=me with a tiny doc comment and green CI. Thanks!

[mati865]

I think this might break windows-gnu targets. Their spec for some reason states pdb debuginfo but they use dwarf. Previously this was handled by is_like_msvc.

rust/compiler/rustc_target/src/spec/base/windows_gnu.rs

Line 102 in 9a1d156

debuginfo_kind: DebuginfoKind::Pdb,

[wesleywiser]

Thanks for pointing that out! That seems super weird. I'll look into that tomorrow.

[lqd]

Weird.

We should document that in both cg_llvm and the target specs themselves. And/or maybe the centralization should be in rustc_target/TargetOptions (but made available via the Session if you still want to, without that logic), since it's both target-specific and has subtle rules (that can contradict the spec fields).