[llvm] [KeyInstr] Use DISubprogram's is-key-instructions-on flag at DWARF emission (PR #144104)

[Orlando Cazalet-Hyams via llvm-commits]

OCHyams wrote:

> Looking good; this is going to stimulate KI paths down buildbots that don't have it compiled in, are there any consequences from that? For example, the test being added, surely all the atomGroup fields are going to be clamped to zero?

Tests in that directory are only run if (LLVM_)EXPERIMENTAL_KEY_INSTRUCTIONS is enabled, so that's not an issue.

https://github.com/llvm/llvm-project/pull/144104