llvm: make LLVM 22 the default

[zhouguangyuan0718]

Summary

• select LLVM 22 when no explicit LLVM version tag is present
• keep LLVM 21 available through the explicit llvm21 tag
• align the LLVM 22-only switch operand API implementation with the new default
• move default CI coverage to LLVM 22, including Fedora 44
• update the README default and Windows pkg-config example

Validation

• go test -count=1 -v ./... with no version tag against LLVM 22.1.8: pass
• go test -count=1 -tags=llvm21 ./... against LLVM 21.1.8: pass
• the no-tag run reports linked LLVM 22.1.8 and exercises TestSwitchCaseValue, TargetMachine options, PassBuilder and IR tests

[fennoai]

Review: default LLVM 21 → 22

This PR cleanly bumps the default LLVM version from 21 to 22. The //go:build partition across the config/switch files is mutually exclusive and complete for modern Go: any single llvmNN tag selects exactly one llvm_config_* file and one switch_* file, and the tagless (default) case correctly resolves to LLVM 22 via the negative-clause fallback. The README, CI matrix, and pkg-config references were all updated consistently, and the new Windows LLVM 22 entry preserves the pinned-SHA256 download-verify pattern.

Security and performance passes found nothing to block (no runtime logic changes). One low-severity build-tag consistency note is inline below.

Notes (non-blocking, out of scope)

• .github/workflows/go.yml (Linux job): wget ... | sudo apt-key add - trusts the LLVM key without fingerprint pinning and uses the deprecated apt-key. Pre-existing (line unchanged by this PR); flagging only for awareness.
• README.markdown "Currently supported" list omits FreeBSD despite FreeBSD cgo flags in the config files. Pre-existing, unrelated to this change.

[fennoai]

[P3] Build-tag consistency: //go:build-only files vs go.mod 1.14

This file's build constraint now uses only //go:build, with no legacy // +build fallback — same for switch_llvm22.go and switch_pre22.go. go.mod declares go 1.14, and Go toolchains before 1.17 ignore //go:build lines entirely. On such a toolchain these constraints would be dropped, causing duplicate symbols (e.g. two GetSwitchCaseValue definitions) and a build failure.

This is a pre-existing condition (the switch/llvm22 files already lacked // +build in the base), not a regression introduced here — and CI pins Go 1.22, so it's invisible to the test matrix. But since this PR is already editing these constraint lines, it's a good moment to make the intent explicit: either add matching // +build lines to these three files (the sibling llvm_config_llvm19/20/21.go still carry both forms), or bump go.mod to go 1.17 and drop the now-dead // +build lines everywhere for a uniform style. Low severity / optional.