Retain ".pragma library" when reformatting JavaScript files

For ".pragma library" there is a special flag in QmlJS::Directives.
We just need to actually set the flag and query it when re-emitting the
code.

Change-Id: Ia3455fda18aba3219b02ecf092bb28987a6ccef5
Fixes: QTCREATORBUG-22326
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

src/libs/qmljs/qmljsdocument.cpp

@@ -265,6 +265,11 @@ public:
         addLocation(line, column);
     }
 
+    void pragmaLibrary() override
+    {
+        isLibrary = true;
+    }
+
     virtual QList<SourceLocation> locations() { return _locations; }
 
     const QString documentPath;