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

[Jeremy Morse via llvm-commits]

https://github.com/jmorse commented:

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?

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