[clang] [Driver] Enable --build-id by default (ENABLE_LINKER_BUILD_ID=ON) (PR #212620)

[Fangrui Song via cfe-commits]

MaskRay wrote:

Thanks for putting this together. However, I don't know the case for flipping the default holds up.

On cost. lld defaults to a BLAKE3 based fast build-id, but but it still costs time. On GNU ld it likely costs more.

GCC parity still holds. `gcc/configure.ac` on master still has `enable_linker_build_id=no`.

Defaults should serve whoever can't easily override. Builds that want a build-id already have a build configuration: a spec file, `debian/rules`, a CI job, a toolchain definition. Builds that don't want it are the inner development loop - many relinks a day, no config file, nobody tuning them. Shifting cost onto that group to save packagers a flag they've already set seems backwards.

I realize the counter is that you often don't know at link time whether you'll want the ID. But you then should explicitly specify --build-id={fast,md5,sha1}.

**More fundamentally, I think the CMake option is the wrong mechanism for distro policy.** A distribution that wants `--build-id` by default can express that in a clang configuration file instead of a compile-time flag. This works cleanly since at least Clang 18. We should try harder to remove such CMake options.

https://github.com/llvm/llvm-project/pull/212620