[clangd] Dont assert on specific uris for diagnostics docs

To enable customization of links in downstream projects without
breaking tests (and also ease of pointing at different links in the future).
Just check for existence instead.

Differential Revision: https://reviews.llvm.org/D157071

GitOrigin-RevId: 734da23e21eb687ff4a613d520cbef5348e26a65
2 files changed