Add QmlJS::Document::ast() for when the type of the AST doesn't matter.

Ideally most of our visitors won't need to make the QML vs JS
distinction. For these cases it'll be handy to be able to access the AST
no matter whether it's a Program or UiProgram.

src/libs/qmljs/qmljsdocument.cpp

@@ -82,6 +82,14 @@ AST::Program *Document::jsProgram() const
     return _jsProgram;
 }
 
+AST::Node *Document::ast() const
+{
+    Q_ASSERT(!_uiProgram || !_jsProgram);
+    if (_uiProgram)
+        return _uiProgram;
+    return _jsProgram;
+}
+
 QList<DiagnosticMessage> Document::diagnosticMessages() const
 {
     return _diagnosticMessages;