C++ insert def: Fix bug when minimizing name.

Constructors would not get the correct name before. Now rewriting the
function's name is not done by going through the 'rewrite type' func-
tionality but rather by minimizing the symbol's name directly.

Task-number: QTCREATORBUG-6223
Change-Id: I3c25e414337937f5dd0f54570c899ca2ca21d2ef
Reviewed-by: Leandro T. C. Melo <leandro.melo@nokia.com>