tree-core: clarify clobber comments

It came up on the mailing list that OBJECT_BEGIN/END are described as
marking object lifetime, but mark the beginning of the constructor and end
of the destructor, whereas the C++ notion of lifetime is between the end of
the constructor and beginning of the destructor.  So let's fix the comments.

gcc/ChangeLog:

	* tree-core.h (enum clobber_kind): Clarify CLOBBER_OBJECT_*
	comments.