[clang-tools-extra] [clangd] Let DefineOutline tweak create a definition from scratch (PR #71950)

[Tom Praschan via cfe-commits]

tom-anders wrote:

> We should probably rekindle the discussion on the associated bug report; I think users (rightly) expect this feature.

Agreed, [maybe a protocol extension would also be an option](https://github.com/clangd/clangd/issues/445#issuecomment-1250355799)

https://github.com/llvm/llvm-project/pull/71950