Do not underline unnecessary diagnostics (#31355)

Closes
https://github.com/zed-industries/zed/pull/31229#issuecomment-2906946881

Follow
https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#diagnosticTag

> Clients are allowed to render diagnostics with this tag faded out
instead of having an error squiggle.

and do not underline any unnecessary diagnostic at all.

Release Notes:

- Fixed clangd's inactive regions diagnostics excessive highlights

crates/project/src/lsp_store/clangd_ext.rs

@@ -10,6 +10,7 @@ use crate::LspStore;
 
 pub const CLANGD_SERVER_NAME: &str = "clangd";
 const INACTIVE_REGION_MESSAGE: &str = "inactive region";
+const INACTIVE_DIAGNOSTIC_SEVERITY: lsp::DiagnosticSeverity = lsp::DiagnosticSeverity::INFORMATION;
 
 #[derive(Debug, Eq, PartialEq, Clone, Deserialize, Serialize)]
 #[serde(rename_all = "camelCase")]
@@ -28,7 +29,7 @@ impl lsp::notification::Notification for InactiveRegions {
 
 pub fn is_inactive_region(diag: &Diagnostic) -> bool {
     diag.is_unnecessary
-        && diag.severity == lsp::DiagnosticSeverity::INFORMATION
+        && diag.severity == INACTIVE_DIAGNOSTIC_SEVERITY
         && diag.message == INACTIVE_REGION_MESSAGE
         && diag
             .source
@@ -59,11 +60,11 @@ pub fn register_notifications(
                         .into_iter()
                         .map(|range| lsp::Diagnostic {
                             range,
-                            severity: Some(lsp::DiagnosticSeverity::INFORMATION),
+                            severity: Some(INACTIVE_DIAGNOSTIC_SEVERITY),
                             source: Some(CLANGD_SERVER_NAME.to_string()),
                             message: INACTIVE_REGION_MESSAGE.to_string(),
                             tags: Some(vec![lsp::DiagnosticTag::UNNECESSARY]),
-                            ..Default::default()
+                            ..lsp::Diagnostic::default()
                         })
                         .collect();
                     let mapped_diagnostics = lsp::PublishDiagnosticsParams {