Allow LSP adapters to decide, which diagnostics to underline (#31450)

Closes
https://github.com/zed-industries/zed/pull/31355#issuecomment-2910439798

<img width="1728" alt="image"
src="https://github.com/user-attachments/assets/2eaa8e9b-00bc-4e99-ac09-fceb2d932e41"
/>


Release Notes:

- N/A

crates/languages/src/c.rs

@@ -285,6 +285,10 @@ impl super::LspAdapter for CLspAdapter {
     fn retain_old_diagnostic(&self, previous_diagnostic: &Diagnostic, _: &App) -> bool {
         clangd_ext::is_inactive_region(previous_diagnostic)
     }
+
+    fn underline_diagnostic(&self, diagnostic: &lsp::Diagnostic) -> bool {
+        !clangd_ext::is_lsp_inactive_region(diagnostic)
+    }
 }
 
 async fn get_cached_server_binary(container_dir: PathBuf) -> Option<LanguageServerBinary> {