c++: cp_tree_equal tweaks

c++: cp_tree_equal tweaks

When comparing streamed trees we can encounter NON_LVALUE_EXPR and
VIEW_CONVERT_EXPRs with null types.  Also, when checking a potential
duplicate we don't want to reject PARM_DECLs with different contexts,
if those two contexts are the two decls of interest.

	gcc/cp/
	* cp-tree.h (map_context_from, map_context_to): Declare.
	* module.cc (map_context_from, map_context_to): Define.
	* tree.c (cp_tree_equal): Check map_context_{from,to} for parm
	context difference.  Allow NON_LVALUE_EXPR and VIEW_CONVERT_EXPR
	with null types.