Ticket #6023: Properly handle template'd default template parameter values

Hi,

This ticket is a (incorrect) syntax error reported because we fail to handle template'd default template parameter values, and make the code invalid when "injecting" such a default value. This patch fixes it by keeping track of the "parameter depth" in a part of the code that did not. Thanks to consider merging.

Cheers,
  Simon

PS: I plan to work on replacing ">>" by "> >" when it makes sense and factorize somehow the "parameter depth" code that's more or less duplicated several times in templatesimplifier.cpp