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

[via cfe-commits]

ckandeler wrote:

> This implements [clangd/clangd#445](https://github.com/clangd/clangd/issues/445)

Aha, so this implementation does not work, because it relies on the AST only for figuring out whether an implementation exists. And querying the index is considered too expensive during the prepare() step. That's a pity then.

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