c++: Improve print_tree of static_assert.

We weren't printing the condition and message of a STATIC_ASSERT.

It's also unnecessary to duplicate the code for instantiating a
STATIC_ASSERT between tsubst_expr and instantiate_class_template_1.

gcc/cp/ChangeLog
2020-05-11  Jason Merrill  <jason@redhat.com>

	* pt.c (instantiate_class_template_1): Call tsubst_expr for
	STATIC_ASSERT member.
	* ptree.c (cxx_print_xnode): Handle STATIC_ASSERT.