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

[Reid Kleckner via llvm-branch-commits]

rnk wrote:

I just rebased, and I noticed I botched the rebase, so I will go back and fix that up before I land.

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