Change compiler’s diagnostic provider name to SourceKit

The fact that they are coming from a service named `sourcekitd` should be an implentation detail of SourceKit-LSP and shouldn’t be exposed to users. Use the generic `SourceKit` term, which is vague about which SourceKit the diagnostics are coming from.

Author: ahoppen

Sources/SourceKitLSP/Swift/Diagnostic.swift

@@ -263,7 +263,7 @@ extension Diagnostic {
       severity: severity,
       code: code,
       codeDescription: codeDescription,
-      source: "sourcekitd",
+      source: "SourceKit",
       message: message,
       tags: tags,
       relatedInformation: notes,
@@ -297,7 +297,7 @@ extension Diagnostic {
       severity: diag.diagMessage.severity.lspSeverity,
       code: nil,
       codeDescription: nil,
-      source: "SwiftSyntax",
+      source: "SourceKit",
       message: diag.message,
       tags: nil,
       relatedInformation: relatedInformation,