Adjust some build troubleshooting notes

[jonmeow]

Came up due to libc++ install issues

We've discussed clang version verification, and adding that as long as I'm in here. The more significant bit is the libc++ check, which if it's not installed should fail like:

(tons of output)
ignoring nonexistent directory "/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/lib/llvm-16/lib/clang/16/include
 /usr/local/include
 /usr/include/x86_64-linux-gnu
 /usr/include
End of search list.
/usr/local/google/home/jperkins/.cache/bazel/_bazel_jperkins/85deb7d9d96f7e0e80b42618a55969d7/external/_main~clang_toolchain_extension~bazel_cc_toolchain/_temp:6:2: error: "No libc++ install found!"
#error "No libc++ install found!"
 ^
1 error generated.
ERROR: Analysis of target '//toolchain:toolchain' failed; build aborted: Analysis failed
INFO: Elapsed time: 0.265s, Critical Path: 0.08s
INFO: 1 process: 1 internal.
ERROR: Build did NOT complete successfully

pre-commit runs bazel, and GitHub runners have an old clang by default (caught by the new check), so I'm installing here for a consistent version.

[danakj]

Can we leave a breadcrumb of the exact error here, so a search will turn it up as well?

Suggestion:

As one example, Bazel may raise "absolute path inclusion(s) found in rule" and "includes the following non-builtin files with absolute paths" as an error for libc++ includes if the location of the standard library header files changes.

[jonmeow]

I'm now tackling the libc++ issue down a different route, just validation during repo setup (also clang versions that we'd discussed in the past)

[chandlerc]

LGTM, but maybe good to double check that with the precise & functional testing of libc++ now this has enough info in the troubleshooting from @danakj's perspective before merging?

[danakj]

LGTM, thanks!