Make QDoc aware of the namespace scope of an \fn command

The Clang-based C++ parser in QDoc construct a temporary translation unit for the function definition from the \fn command parameter. Currently however, this mechanism ignores any
namespace under which the \fn-containing comment is located; this forces the writer to provide fully qualified paths for everything in the function signature.

Tracking the current namespace scope would make writing \fn command more intuitive and improve the reliability of matching the function documentation to the correct node from the header.