Change meaning of VALUE_FRAME_ID; rename to VALUE_NEXT_FRAME_ID

Change meaning of VALUE_FRAME_ID; rename to VALUE_NEXT_FRAME_ID

The VALUE_FRAME_ID macro provides access to a member in struct value
that's used to hold the frame id that's used when determining a
register's value or when assigning to a register.  The underlying
member has a long and obscure name.  I won't refer to it here, but
will simply refer to VALUE_FRAME_ID as if it's the struct value member
instead of being a convenient macro.

At the moment, without this patch in place, VALUE_FRAME_ID is set in
value_of_register_lazy() and several other locations to hold the frame
id of the frame passed to those functions.

VALUE_FRAME_ID is used in the lval_register case of
value_fetch_lazy().  To fetch the register's value, it calls
get_frame_register_value() which, in turn, calls
frame_unwind_register_value() with frame->next.

A python based unwinder may wish to determine the value of a register
or evaluate an expression containing a register.  When it does this,
value_fetch_lazy() will be called under some circumstances.  It will
attempt to determine the frame id associated with the frame passed to
it.  In so doing, it will end up back in the frame sniffer of the very
same python unwinder that's attempting to learn the value of a
register as part of the sniffing operation.  This recursion is not
desirable.

As noted above, when value_fetch_lazy() wants to fetch a register's
value, it does so (indirectly) by unwinding from frame->next.

With this in mind, a solution suggests itself:  Change VALUE_FRAME_ID
to hold the frame id associated with the next frame.  Then, when it
comes time to obtain the value associated with the register, we can
simply unwind from the frame corresponding to the frame id stored in
VALUE_FRAME_ID.  This neatly avoids the python unwinder recursion
problem by changing when the "next" operation occurs.  Instead of the
"next" operation occuring when the register value is fetched, it
occurs earlier on when assigning a frame id to VALUE_FRAME_ID.
(Thanks to Pedro for this suggestion.)

This patch implements this idea.

It builds on the patch "Distinguish sentinel frame from null frame".
Without that work in place, it's necessary to check for null_id at
several places and then obtain the sentinel frame.

It also renames most occurences of VALUE_FRAME_ID to
VALUE_NEXT_FRAME_ID to reflect the new meaning of this field.

There are several uses of VALUE_FRAME_ID which were not changed.  In
each case, the original meaning of VALUE_FRAME_ID is required to get
correct results.  In all but one of these uses, either
put_frame_register_bytes() or get_frame_register_bytes() is being
called with the frame value obtained from VALUE_FRAME_ID.  Both of
these functions perform some unwinding by performing a "->next"
operation on the frame passed to it.  If we were to use the new
VALUE_NEXT_FRAME_ID macro, this would effectively do two "->next"
operations, which is not what we want.

The VALUE_FRAME_ID macro has been redefined in terms of
VALUE_NEXT_FRAME_ID.  It simply fetches the previous frame's id,
providing this id as the value of the macro.

gdb/ChangeLog:

	* value.h (VALUE_FRAME_ID): Rename to VALUE_NEXT_FRAME_ID. Update
	comment.  Create new VALUE_FRAME_ID which is defined in terms of
	VALUE_NEXT_FRAME_ID.
	(deprecated_value_frame_id_hack): Rename to
	deprecated_value_next_frame_id_hack.
	* dwarf2loc.c, findvar.c, frame-unwind.c, sentinel-frame.c,
	valarith.c, valops.c, value.c: Adjust nearly all occurences of
	VALUE_FRAME_ID to VALUE_NEXT_FRAME_ID.	Add comments for those
	which did not change.
	* value.c (struct value): Rename frame_id field to next_frame_id.
	Update comment.
	(deprecated_value_frame_id_hack): Rename to
	deprecated_value_next_frame_id_hack.
	(value_fetch_lazy): Call frame_unwind_register_value()
	instead of get_frame_register_value().
	* frame.c (get_prev_frame_id_by_id): New function.
	* frame.h (get_prev_frame_id_by_id): Declare.
	* dwarf2loc.c (dwarf2_evaluate_loc_desc_full): Make
	VALUE_NEXT_FRAME_ID refer to the next frame.
	* findvar.c (value_of_register_lazy): Likewise.
	(default_value_from_register): Likewise.
	(value_from_register): Likewise.
	* frame_unwind.c (frame_unwind_got_optimized): Likewise.
	* sentinel-frame.c (sentinel_frame_prev_register): Likewise.
	* value.h (VALUE_FRAME_ID): Update comment describing this macro.