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

[Pavel Labath via cfe-commits]

labath wrote:

Ideally, I'd also go with the uniform approach of enabling the feature everywhere. It's true that this doesn't have any value for lldb right now, but it also shouldn't break anything (lldb ignores `is_stmt`), and it's not really true that this (disabling the flag on -glldb) will completely avoid having to deal with this input as it's fairly common to compile linux binaries with -ggdb (the default) and debug with lldb.

Enabling the flag may also provide the incentive for someone to add support for this to lldb (I wish that someone was me, but it looks like that's not going to happen). Even if that happens a year from now, it would be nice if we could say that all binaries compiled within the last year would benefit from that.

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