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

[via cfe-commits]

argothiel wrote:

These are two different things:

VS Code always advertises `insertReplaceSupport: true`. It means that as an LSP client, it can support both modes: a simple TextEdit range, or a more convoluted InsertReplaceEdit range.

clangd as an LSP server has always been sending a simple TextEdit range. And `vscode-clangd` has always been expecting a simple TextEdit range.

Now, if an LSP server doesn't support an InsertReplaceEdit range, then VS Code will use the same TextEdit range as both replace and insert, regardless of the `editor.suggest.insertMode` setting. That's why the `vscode-clangd` plugin has been working fine.

But when the LSP server supports an InsertReplaceEdit range, it will always send it to the client as a two-field structure (insert and replace), which is handled fine by VS Code, but is not handled correctly by one of the `vscode-clangd`'s methods. Only after receiving both insert and replace range, VS Code will decide which one to use depending on the `editor.suggest.insertMode` setting.

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