Improve support for resolving enums in default argument expressions.

One place in Qt5WebEngineWidgets has a method with this signature:

void printToPdf(const QString &filePath, const QPageLayout &layout = QPageLayout(QPageSize(QPageSize::A4), QPageLayout::Portrait, QMarginsF()));

The DefaultArgVisitor was providing incorrect results for that default
argument. It saw the two enums and would just concatenate them together.
A better approach is to use the clang Rewriter class to insert the parent
namespace/class into the expression, and return the rewritten text after
the expression has been fully traversed.