[clang] [llvm] [KeyInstr] Add DISubprogram::keyInstructions bit (PR #144107)

[Jeremy Morse via cfe-commits]

https://github.com/jmorse approved this pull request.

LGTM, I think this raises a few questions about whether we're going to support non-key-instructions code forever more. AFAIUI the premise is that the debugging behaviour is worse if you've got groupless instructions in a key-instructions program? There might be various routes that introduce these things:
 * Old bitcode being autoupgraded,
 * Developers who turn the option off,
 * LLVM frontends that don't support / implement key instructions,

Which over time will hopefully reduce / change / be implemented. Presumably in some future date we'll be able to make key instructions non-optional. Can you forsee us doing anything differently in anticipation of that?

One wonders whether DebugKeyInstructions should be a mandatory non-default argument, to let people know that something significant is changing, and meaning that we don't have to bake the default into all the callsites. On the other hand, it's not like this is a common or frequently used API call.

arg to DISubprograms isn't defaultable, so we shouldn't expand everything?

https://github.com/llvm/llvm-project/pull/144107