PR c++/54466 - ICE with alias template which type-id is const qualified

Consider this short example:

    template<typename T>
      struct X { };

    template<typename T>
      using Y = const X<T>;

    using Z = Y<int>;

G++ crashes in lookup_class_template_1 while trying to build the alias
template instantiation Y<int>.

I think this is indirectly due to the fact that that
lookup_class_template_1 can now yield a const qualified type like
'const X<T>'.

As a consequence, the code in lookup_template_class_1 that was trying
to access the TYPE_STUB_DECL field of the result of
lookup_template_class_1 should now be adjusted to access the
TYPE_STUB_DECL of the main variant of the resulting type instead (and
that is TYPE_MAIN_DECL); because qualified types (constructed with
build_qualified_type) have their TYPE_STUB_DECL set to NULL.

Fixed thus and tested on x86_64-unknown-linux-gnu against trunk.

gcc/cp

	PR c++/54466
	* pt.c (lookup_template_class_1): TYPE_STUB_DECL should be
	accessed on the main variant of the type.

gcc/testsuite/

	* g++.dg/cpp0x/alias-decl-26.C: New test file.

In the example of this patch, g++ crashes when trying to build the
alias template Y<int

From-SVN: r193479