[SourceKit] Allow module references to be indexed (#19243)

Module references get indexed as a 'module' symbol; they get USRs similar to how clang would assign a USR for a module reference.

JIRA: https://bugs.swift.org/browse/SR-8677

tools/SourceKit/lib/SwiftLang/SwiftLangSupport.cpp

@@ -583,6 +583,9 @@ UIdent SwiftLangSupport::getUIDForSymbol(SymbolInfo sym, bool isRef) {
       llvm_unreachable("missing extension sub kind");
     }
 
+  case SymbolKind::Module:
+    return KindRefModule;
+
   default:
     // TODO: reconsider whether having a default case is a good idea.
     return UIdent();