libstdc++/7186: DR179 for std::deque::iterator and const_iterator

[Paolo Carlini]

A little update on the existing practice (as requested by Benjamin):
- Interestingly, Intel6 (Dinkumware) and Borland5.5.1 (Rogue Wave) show 
a behaviour identical to that of current mainline v3 that is 6642's 
testcase is compiled just fine, while 7186's testcase is not.
- Metrowerks8 is able to compile both (this is the behaviour mandated by 
the resolution of DR179, according to my reading)

Paolo.

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=7186