Move compilation unit info to dwarf_expr_context

Move compilation unit info to dwarf_expr_context

This patch moves the compilation unit context information and support
from dwarf_expr_executor and dwarf_evaluate_loc_desc to
dwarf_expr_context evaluator. The idea is to report an error when a
given operation requires a compilation unit information to be resolved,
which is not available.

With this change, it also makes sense to always acquire ref_addr_size
information from the compilation unit context, considering that all
DWARF operations that refer to that information require a compilation
unit context to be present during their evaluation.

gdb/ChangeLog:

	* dwarf2/expr.c (ensure_have_per_cu): New function.
	(dwarf_expr_context::dwarf_expr_context): Add compilation unit
	context information.
	(dwarf_expr_context::get_base_type): Move from
	dwarf_evaluate_loc_desc.
	(dwarf_expr_context::get_addr_index): Remove method.
	(dwarf_expr_context::dwarf_variable_value): Remove method.
	(dwarf_expr_context::execute_stack_op): Call compilation unit
	context info check. Inline get_addr_index and
	dwarf_variable_value methods.
	* dwarf2/expr.h (struct dwarf_expr_context): Add compilation
	context info.
        (dwarf_expr_context::get_addr_index): Remove method.
        (dwarf_expr_context::dwarf_variable_value): Remove method.
        (dwarf_expr_context::ref_addr_size): Remove member.
	* dwarf2/frame.c (dwarf_expr_executor::get_addr_index): Remove
	method.
	(dwarf_expr_executor::dwarf_variable_value): Remove method.
	* dwarf2/loc.c (sect_variable_value): Expose function.
	(dwarf_evaluate_loc_desc::get_addr_index): Remove method.
	(dwarf_evaluate_loc_desc::dwarf_variable_value): Remove method.
	(class dwarf_evaluate_loc_desc): Move compilation unit context
	information to dwarf_expr_context class.
	* dwarf2/loc.h (sect_variable_value): Expose function.