[SourceKit] Stop using isSystemModule to represent "non-user" modules

Rather than using `ModuleDecl::isSystemModule()` to determine whether a
module is not a user module, instead check whether the module was
defined adjacent to the compiler or if it's part of the SDK.

If no SDK path was given, then `isSystemModule` is still used as a
fallback.

Resolves rdar://89253201.

lib/IDE/CodeCompletionStringPrinter.cpp

@@ -131,7 +131,7 @@ void CodeCompletionStringPrinter::printTypeRef(Type T, const TypeDecl *TD,
                                                Identifier Name,
                                                PrintNameContext NameContext) {
 
-  NextChunkKind = TD->getModuleContext()->isSystemModule()
+  NextChunkKind = TD->getModuleContext()->isNonUserModule()
                       ? ChunkKind::TypeIdSystem
                       : ChunkKind::TypeIdUser;