[clang] [KeyInstr] Enable -gkey-instructions by default if optimisations are enabled (PR #149509)

[Orlando Cazalet-Hyams via cfe-commits]

OCHyams wrote:

Thanks all for the comments!

In addition to the cc'd apple folks, @labath do you have an opinion on the topic of enabling this by default for all debuggers versus not doing so for LLDB tuning? (There's a question in the summary, and it's been discussed a little in the comments).

One less exciting question that remains, if anyone has any ideas off the top of their heads: 
> Clang languages/extensions - I've implemented support for C/C++, but I've not tested or specifically implemented anything for other languages (?) or extensions (?) like CUDA. Is there a good way to filter for this in the driver code in this patch?

https://github.com/llvm/llvm-project/pull/149509