This patch adds a Clang-compatible --save-stats option to opt, to
provide an easy to use way to save LLVM statistics files when working
with opt on the middle end.
This is a follow up on the addition to `llc`:
https://github.com/llvm/llvm-project/pull/163967
Like on Clang, one can specify --save-stats, --save-stats=cwd, and
--save-stats=obj with the same semantics and JSON format. The
pre-existing --stats option is not affected.
The implementation extracts the flag and its methods into the common
`CodeGen/CommandFlags` as `LLVM_ABI`, using a new registration class to
conservatively enable opt-in rather than let all tools take it. Its only
needed for llc and opt for now. Then it refactors llc and adds support
for opt.
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