Richard Smith proposed adding a synonym for __is_same_as, to accomodate the
convention of exposing std::SOME_TRAIT<A, B>::value as __SOME_TRAIT(A, B).

So add that alias, and also adjust the C++ printer.  I didn't bother changing
the RID_ identifier.

	* c-common.c: Add __is_same, an alias for __is_same_as.

	* cxx-pretty-print.c (pp_cxx_trait_expression) <case CPTK_IS_SAME_AS>:
	Print "__is_same".

	* g++.dg/ext/is_same.C: New test.

From-SVN: r279018