c++: alias template equivalence and cv-quals [PR100032]

We also need to check that the cv-qualifiers are the same.

gcc/cp/ChangeLog:

	PR c++/100032
	* pt.c (get_underlying_template): Compare TYPE_QUALS.

gcc/testsuite/ChangeLog:

	PR c++/100032
	* g++.dg/cpp0x/alias-decl-equiv1.C: New test.