Add dwarf2_per_objfile to dwarf_expr_context and dwarf2_frame_cache

Add dwarf2_per_objfile to dwarf_expr_context and dwarf2_frame_cache

Evaluating DWARF expressions (such as location expressions) requires
knowing about the current objfile.  For example, it may call functions
like dwarf2_fetch_die_loc_sect_off, which currently obtain the
dwarf2_per_objfile object it needs from the dwarf2_per_cu_data object.
However, since we are going to remove this
dwarf2_per_cu_data::dwarf2_per_objfile link, these functions will need
to obtain the current dwarf2_per_objfile by parmeter.

If we go up the stack, we see that the DWARF expression contexts
(dwarf_expr_context and the classes that derive from it) need to store
the dwarf2_per_objfile, to be able to pass it to those functions that
will need it.

This patch adds a constructor to all these dwarf_expr_context variants,
accepting a dwarf2_per_objfile parameter.  This dwarf2_per_objfile
generally comes from a symbol baton created earlier.

For frame-related expressions, the dwarf2_per_objfile object must be
passed through the dwarf2_frame_cache object.  This lead to the
dwarf2_frame_find_fde function returning (by parameter) a
dwarf2_per_objfile object.  I then realized that this made the existing
"out_offset" parameter redundant.  This offset is
`objfile->text_section_offset ()`, so it can be recomputed from the
dwarf2_per_objfile object at any time.  I therefore opted to remove this
output parameter, as well as the offset field of dwarf2_frame_cache.

*Note*, there's one spot I'm particularly unsure about.  In
dwarf_evaluate_loc_desc::push_dwarf_reg_entry_value, we would save and
overwrite the offset value in the context, along with a bunch of other
state.  This is because we might be about to evaluate something in a
different CU that the current one.  If the two CUs are in the same
objfile, then the text_offset is the same, as it's a property of the
objfile.  However, if the two CUs are possibly in different objfiles,
then it means the text_offsets are different.  It would also mean we
would need to save and restore the dwarf2_per_objfile in the context.
Is that even possible?

gdb/ChangeLog:

	* dwarf2/expr.h (struct dwarf_expr_context)
	<dwarf_expr_context>: Add dwarf2_per_objfile parameter.
	<offset>: Remove.
	<per_objfile>: New member.
	* dwarf2/expr.c (dwarf_expr_context::dwarf_expr_context): Add
	dwarf2_per_objfile parameter.  Don't set offset, set
	per_objfile.
	(dwarf_expr_context::execute_stack_op): Use offset from objfile.
	* dwarf2/frame.c (dwarf2_frame_find_fde): Return (by parameter)
	a dwarf2_per_objfile object instead of an offset.
	(class dwarf_expr_executor) <dwarf_expr_executor>: Add
	constructor.
	(execute_stack_op): Add dwarf2_per_objfile parameter, pass it
	to dwarf2_expr_executor constructor.  Don't set offset.
	(dwarf2_fetch_cfa_info): Update.
	(struct dwarf2_frame_cache) <text_offset>: Remove.
	<per_objfile>: New field.
	(dwarf2_frame_cache): Update.
	(dwarf2_frame_prev_register): Update.
	* dwarf2/loc.c (class dwarf_evaluate_loc_desc)
	<dwarf_evaluate_loc_desc>: Add constructor.
	(dwarf2_evaluate_loc_desc_full): Update.
	(dwarf2_locexpr_baton_eval): Update.
	(class symbol_needs_eval_context) <symbol_needs_eval_context>:
	Add constructor.
	(dwarf2_loc_desc_get_symbol_read_needs): Update.

Change-Id: I14df060669cc36ad04759f1708c6d7b1fda77727