Add deprecated tag to diagnostics (#660)

This maps nicely to the LSP DiagnosticTag.Deprecated enum value.

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

Author: stefanvanburen

experimental/ir/lower_deprecated.go

@@ -16,6 +16,7 @@ package ir
 
 import (
 	"github.com/bufbuild/protocompile/experimental/report"
+	"github.com/bufbuild/protocompile/experimental/report/tags"
 	"github.com/bufbuild/protocompile/experimental/seq"
 	"github.com/bufbuild/protocompile/experimental/source"
 )
@@ -128,5 +129,6 @@ func (e errDeprecated) Diagnose(d *report.Diagnostic) {
 		report.Message("`%s` is deprecated", e.name),
 		report.Snippet(e.ref),
 		report.Snippetf(e.cause, "deprecated here"),
+		report.Tag(tags.Deprecated),
 	)
 }

experimental/report/tags/tags.go

@@ -25,4 +25,7 @@ const (
 	// in the current scope. This may indicate a missing import or a reference
 	// to a non-existent symbol.
 	UnknownSymbol = "protobuf:unknown_symbol"
+
+	// Deprecated is the tag for a diagnostic where a symbol is deprecated.
+	Deprecated = "protobuf:deprecated"
 )