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

[David Blaikie via cfe-commits]

dwblaikie wrote:

A summary of DWARF file size/section size changes (how much change as a % of total file size in object and linked executables, as a % of aggregate DWARF section sizes in object and linked executables, and as a % of the specific DWARF section sizes (eg: {.rela,}.debug_line changes by x%) - with specific debug info flags (my preference would be for -fno-standalone-debug -gz=zstd with crel, but I understand if that's not your deployment target, etc, it's less relevant)) and which debuggers are effected in what ways (some discussion of gdb would be nice).

If this has a cost and no benefit for lldb, seems like maybe we'd want to keep it off under lldb tuning. If it has a cost and no benefit to gdb - then I'd probably say off-by-default, on-for-SCE.

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