`llvm.download-ci-llvm = true` + `assertions = true` with unavailable alt CI LLVM shows no warnings/errors, tries to build LLVM

Summary

On hosts (e.g. aarch64-apple-darwin) where we don't built "alt CI LLVM" (CI LLVM with assertions enabled), a configuration like

[llvm]
download-ci-llvm = true
assertions = true

will fail to find a CI LLVM from the alternative channel that has "compatible LLVM config" (namely with assertions enabled), yet prints zero warnings/errors, and then tries to checkout llvm submodule and build it locally.

When download-ci-llvm = true, the user intention is to force usage of CI LLVM. We should at least show a warning here, or error, and not silently try to build LLVM.

Command used

./x build library

Expected behaviour

Warning or error for incompatible LLVM config versus available CI LLVM artifacts

Actual behaviour

0 errors 0 warnings

Bootstrap configuration (bootstrap.toml)

[llvm]
download-ci-llvm = true
assertions = true

Operating system

aarch64-apple-darwin host

HEAD

a04c7a037d59b6dedc9f921e84e391f13a257aa4

Additional context

N/A

Start with bootstrap.toml's [llvm] settings and run ./x build library using download-ci-llvm = true and assertions = true on the reported host. Trace CI LLVM selection for a compatible artifact, then ensure the unavailable alternative channel produces the requested warning or error instead of silently building LLVM locally.