x/build: add a Linux LUCI builder with a recent Clang toolchain

[bcmills]

As noted in #64423 (comment), there is a cmd/go test that currently fails with Clang 14.0.6, but passes on the linux-amd64-clang post-commit builder (https://build.golang.org/log/f99f0dad3b4c5e7ad8da042b9f541a02313edc02), presumably because the clang version installed on that builder is very old (7.0.1, released December 2018) and reports its version differently.

It appears that the most recent Clang release is 17.0.6, released Nov. 28, 2023.

We expect Go users to upgrade to the latest supported release within ~one year. We should at least test against our own dependencies with the same promptness, which would imply a builder with at least least Clang 15.

[qiulaidongfeng]

See #64400 , cmd/cgo/internal/testtls if the failure is not Windows-specific, the new builder may see the same failure. But I haven't tried to test it on linux.

[gopherbot]

Change https://go.dev/cl/559255 mentions this issue: main.star: add clang16 builder type

[mknyszek]

Assuming that all works well with this attempt to add a clang builder to LUCI, I've also sent out a CL on documentation on how to add new builders for new clang versions and how to update clang minor versions. It should be easy after this to just add as many clang builders as we want, for whatever version we want to test.

[dmitshur]

which would imply a builder with at least least Clang 15

There's a LUCI builder with clang 15 (e.g., gotip-linux-amd64-clang15) and it's been working okay for a while, so I think we can close this out. (There were some additional issues uncovered initially, and I believe they're resolved by now. A separate issue #65356 tracks more work for future clang 16+ builders.)

Thanks @mknyszek.