[helly25]

Cleanup of expression-based LLVM and linker version selectors using bzl 0.8.0, with independent llvm_version_allow_prereleases and linker_version_allow_prereleases controls. This clean redo supersedes the closed #909.

Mainly we are using the advanced/generalized bzl 0.8.0 functionality.

NOTE: The two *_allow_prereleases are incompatible changes. They replace the old magic allowance by referencing in requirements.

AG;DR

• Share requirement parsing, SemVer comparisons, projection constants, filtering, and first/latest selection through bzl. Restrict candidates to compatible artifacts before selecting and preserve the chosen artifact alongside its version.
• Both public prerelease flags default to false. Requirements always apply; exact pins remain authoritative. The generic allow_prereleases parameter exists only in internal resolver code and tests.
• Use the same environment selector expansion for LLVM and catalogued linkers, including mold. Feature overrides inherit the flags independently; linker-only changes preserve LLVM archive sharing.
• Replace _version_string with bzl normalize_string; existing distribution goldens remain unchanged.
• Resolve bzl 0.8.0 directly from BCR in all four module roots, with no temporary overrides. Both WORKSPACE declarations use the published release archive. mboworks_bzl@0.8.0 bazelbuild/bazel-central-registry#11173 is merged; the release dependency is satisfied.
• Validation at synchronized head c71ba5c: all 45 toolchain tests passed in Bzlmod (Bazel 9.3.0) and all 45 passed in WORKSPACE mode (Bazel 8.8.1). All 14 Python wrapper tests and 11 distribution-script tests passed; Trunk buildifier/prettier checks passed. The three integration module graphs were previously validated at 1e1ea62 and were not rerun for this sync.
• Remove the redundant distribution-version wrapper; both production callers use the existing parser directly.
• Move all three LLVM distribution golden-output writers and their fixtures into llvm_distributions_test.bzl. Production code exposes one internal-only testing struct with eight helper references; existing test targets and golden files are unchanged.
• Tests cover selector parity, prerelease pins, platform compatibility, environment fallbacks and errors, and independent feature-override policies. The complete bzl 0.8.0 dependency upgrade merged into master via Update dependency mboworks_bzl to v0.8.0 #912 (including Update Bzlmod dependency to mboworks_bzl 0.8.0 #913). This branch is synchronized with upstream master 6ceed3a, including the dylib-linked tool wrapper fix (Fix remote execution of dylib-linked LLVM tools #914) and LLVM 23.1.4 catalogue update (Update LLVM distributions to 23.1.4 #915). The prerelease golden conflict was resolved by preserving all expanded selector cases and updating only the catalogue-driven expectations to 23.1.4; synthetic fixtures are unchanged. Its diff against master contains only the selector functionality, tests, and documentation, plus removal of an obsolete dependency comment.