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/language/src/buffer.rs

@@ -492,7 +492,7 @@ pub struct Diff {
     pub edits: Vec<(Range<usize>, Arc<str>)>,
 }
 
-#[derive(Clone, Copy)]
+#[derive(Debug, Clone, Copy)]
 pub(crate) struct DiagnosticEndpoint {
     offset: usize,
     is_start: bool,
@@ -4592,7 +4592,7 @@ impl<'a> Iterator for BufferChunks<'a> {
                 syntax_highlight_id: highlight_id,
                 diagnostic_severity: self.current_diagnostic_severity(),
                 is_unnecessary: self.current_code_is_unnecessary(),
-                ..Default::default()
+                ..Chunk::default()
             })
         } else {
             None