[llvm] [CMake] Options to control generation of reproducers (PR #143037)

[Petr Hosek via llvm-commits]

petrhosek wrote:

We've had the equivalent in [Fuchsia's build](https://cs.opensource.google/fuchsia/fuchsia/+/main:build/config/BUILD.gn;l=85-98) and we enable this option on all our builders (including the error reproducers). This has been invaluable and I'd like have the same for our downstream toolchain builders, but I can imagine these might be also helpful for the upstream LLVM builders, especially if we could make the reproducers available as workflow artifacts.

https://github.com/llvm/llvm-project/pull/143037