[clang] [clang][codegen] Mention the invariant that LLVM demangler should be … (PR #117346)

[David Blaikie via cfe-commits]

dwblaikie wrote:

(please don't merge PRs that haven't been approved, that's against LLVM practices/policies ( https://llvm.org/docs/CodeReview.html#code-review-workflow ), unless they weren't intended for pre-commit review in the first place (if they were only meant for presubmit checks))

https://github.com/llvm/llvm-project/pull/117346