c++: Improve build_template_decl

c++: Improve build_template_decl

I discovered all the users of build_template_decl were explicitly
setting the RESULT and TYPE fields of the built decl.  Let's just have
build_template_decl do that in the first place.

	* pt.c (build_template_decl): Init DECL_TEMPLATE_RESULT &
	TREE_TYPE here ...
	(process_partial_specialization): ... not here ...
	(push_template_decl_real, add_inherited_template_parms)
	(build_deduction_guide): ... or here.