[llvm] [KeyInstr] Enable Key Instructions support in LLVM (PR #144324)

[Jeremy Morse via llvm-commits]

https://github.com/jmorse approved this pull request.

See Orlandos discourse post -- I believe we've rustled up sufficient memory-savings to justify landing this, overall users will see reduced max-rss for debug-info builds between LLVM20 and LLVM21.

Will land this shortly, after a final local-test passes. This is an easy revert if it causes any trouble.

https://github.com/llvm/llvm-project/pull/144324