c++: Allow references to internal-linkage vars in C++11 [PR113266]

[temp.arg.nontype] changed in C++11 to allow naming internal-linkage
variables and functions.  We currently already handle internal-linkage
functions, but variables were missed; this patch updates this.

	PR c++/113266
	PR c++/116911

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_template_argument): Allow
	internal-linkage variables since C++11.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/nontype6.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>