[clang-tools-extra] [clangd] Allow "move function body out-of-line" in non-header files (PR #69704)

[via cfe-commits]

ckandeler wrote:

> Would you mind evolving the patch (and the infra) to work for method-definitions inside anon namespaces?

Of course not; I didn't even now it wasn't working. 
I suppose the main point is to determine the insertion location such that it is still within the original namespace?
(I haven't looked at that part of the code so far.)

https://github.com/llvm/llvm-project/pull/69704