[llvm] [llvm-objcopy][COFF] Update .symidx values after stripping (PR #153322)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

I'd like to have this fix in 21.x; https://github.com/llvm/llvm-project/commit/be5c96bfac328fed548c532bbe1710fe23460a85 caused the issue of strip corrupting object files built with CFGuard, in cases where it didn't use to before - see https://github.com/curl/curl-for-win/commit/a5e6d0dcffa55a3da4ddd346c7ca77f05e455ece for discussion/debugging around that. With this patch, that issue is fixed.

So I'll propose backporting it.

https://github.com/llvm/llvm-project/pull/153322