c++: cache partial template specialization selection

There's currently no cheap way to obtain the partial template
specialization (and arguments relative to it) that was selected for a
class or variable template specialization.  Our only option is to
compute the result from scratch via most_specialized_partial_spec.

For class templates this isn't really an issue because we usually need
this information just once, upon instantiation.  But for variable
templates we need it upon specialization and also later upon instantiation.
We could implement an ad-hoc cache for variable templates only, but it'd
be nice for this information to be readily available in general.

To that end, this patch adds a TI_PARTIAL_INFO field to TEMPLATE_INFO
that holds another TEMPLATE_INFO consisting of the partial template and
arguments relative to it, which most_specialized_partial_spec then
uses to transparently cache its (now TEMPLATE_INFO) result.

Similarly, there's no easy way to go from the DECL_TEMPLATE_RESULT of a
partial TEMPLATE_DECL back to that TEMPLATE_DECL.  (Our best option is to
walk the DECL_TEMPLATE_SPECIALIZATIONS list of the primary TEMPLATE_DECL.)
So this patch also uses this new field to link these entities in both
directions.

gcc/cp/ChangeLog:

	* cp-tree.h (tree_template_info::partial): New data member.
	(TI_PARTIAL_INFO): New tree accessor.
	(most_specialized_partial_spec): Add defaulted bool parameter.
	* module.cc (trees_out::core_vals) <case TEMPLATE_INFO>: Stream
	TI_PARTIAL_INFO.
	(trees_in::core_vals) <case TEMPLATE_INFO>: Likewise.
	* parser.cc (specialization_of): Adjust after making
	most_specialized_partial_spec return TEMPLATE_INFO instead
	of TREE_LIST.
	* pt.cc (process_partial_specialization): Set TI_PARTIAL_INFO
	of 'decl' to point back to the partial TEMPLATE_DECL.  Likewise
	(and pass rechecking=true to most_specialization_partial_spec).
	(instantiate_class_template): Likewise.
	(instantiate_template): Set TI_PARTIAL_INFO to the result of
	most_specialization_partial_spec after forming a variable
	template specialization.
	(most_specialized_partial_spec): Add 'rechecking' parameter.
	Exit early if the template is not primary.  Use the TI_PARTIAL_INFO
	of the corresponding TEMPLATE_INFO as a cache unless 'rechecking'
	is true.  Don't bother setting TREE_TYPE of each TREE_LIST.
	(instantiate_decl): Adjust after making
	most_specialized_partial_spec return TEMPLATE_INFO instead of
	TREE_LIST.
	* ptree.cc (cxx_print_xnode) <case TEMPLATE_INFO>: Dump
	TI_PARTIAL_INFO.

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