Split all the structures, enums, typedefs and some fundamental declarations from tree.h into tree-core.h.

Split all the structures, enums, typedefs and some fundamental
declarations from tree.h into tree-core.h.  Everything else stays in
tree.h for now.

As we convert gimple files, we'll move declarations out of tree.h into
other headers and rewrite missing functions in the new gimple API.

	* Makefile.in (TREE_CORE_H): Define.
	(TREE_H): Use.
	(GTFILES): Add tree-core.h.
	* builtins.c (built_in_class_names): Use BUILT_IN_LAST to
	size the array.
	* tree-core.h: New file.
	Move all data structures, enum, typedefs, global
	declarations and constants from ...
	* tree.h: ... here.

From-SVN: r202172