c++: replace in_template_function

All uses of in_template_function except for the one in cp_make_fname_decl
seem like they could be generalized to consider any template context.
To that end this patch replaces the predicate with a generalized
in_template_context predicate that returns true if we're inside any
template context.  If we legitimately need to consider only function
contexts, as in cp_make_fname_decl, we can just additionally check e.g.
current_function_decl.

One concrete benefit of this, which the adjusted testcase below
demonstrates, is that we no longer instantiate/odr-use entities based on
uses within a non-function template.

gcc/cp/ChangeLog:

	* class.cc (build_base_path): Check in_template_context instead
	of in_template_function.
	(resolves_to_fixed_type_p): Likewise.
	* cp-tree.h (in_template_context): Define.
	(in_template_function): Remove.
	* decl.cc (cp_make_fname_decl): Check current_function_decl
	and in_template_context instead of in_template_function.
	* decl2.cc (mark_used): Check in_template_context instead of
	in_template_function.
	* pt.cc (in_template_function): Remove.
	* semantics.cc (enforce_access): Check in_template_context
	instead of current_template_parms directly.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Waddress-of-packed-member2.C: No longer expect a()
	to be marked as odr-used.