[clang-tools-extra] [clangd] use existing function for code locations in the scopify enum tweak (PR #88737)

[Julian Schmidt via cfe-commits]

5chmidti wrote:

@ckandeler while opening up your other changes in my editor, I wanted to see if some of the logic around `Position`s and `Offset`s could be simplified. It looks like `clangd` already implements some of the functionality in the `SourceCode.h` file. This shouldn't actually change the behavior, so I didn't add a release note.

https://github.com/llvm/llvm-project/pull/88737