[llvm-branch-commits] [llvm] [docs] Rewrite LangRef.md as Markdown (PR #201975)

[Nick Desaulniers via llvm-branch-commits]

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

The rendered output LGTM; I didn't spot any major rendering issues.

https://github.com/llvm/llvm-project/pull/201975