[clang] [clang-tools-extra] [clangd] Add InsertReplaceEdit for code completion (PR #187623)

[Aleksandr Platonov via cfe-commits]

ArcsinX wrote:

> VS Code client is fine, it supports both versions; the gap is in the vscode-clangd plugin, and only in the `serverCompletionRanking` workaround path. I thought of fixing that first, but it's not a bug per se, more like a missing functionality needed only if this PR gets merged

This seems to be a fairly common case when a user uses `clangd` from the `main` branch.
If this PR is merged before the client-side changes, the completion will stop working for no apparent reason: it's practically impossible to figure out that `serverCompletionRanking` needs to be disabled (theoretically this is still possible to understand by checking `Extension host` output, see exception there and start extension debugging, but I bet nobody will do this).
Besides `vscode-clangd`, there's also `coc-clangd`, which also mentions this option https://github.com/clangd/coc-clangd/blob/master/src/config.ts#L57, but it doesn't seem to be used anywhere, so it seems like only the changes in `vscode-clangd` are sufficient.

https://github.com/llvm/llvm-project/pull/187623