Avoid building DIE -> parent DIE map when analyzing a C binary

Avoid building DIE -> parent DIE map when analyzing a C binary

The C language doesn't support namespaces.  So, in that language, the
context of a type is always the global scope.  Hence, the context of a
type DIE is always the global context associated to the current
translation unit.

Thus, when we are analyzing a C binary, we can do away with building
the DIE -> DIE parent map that we later use to get the parent of a
type DIE when we need to determine the context of a given type DIE.

This can save significant time and space.

This patch implements this optimization.

	* include/abg-ir.h (global_scope_sptr): Make this be a share_ptr
	of scope_decl, not of global_scope.
	(translation_unit::get_global_scope): Return a reference to
	scope_decl_sptr.
	* src/abg-ir.cc (translation_unit::get_global_scope): Return a
	scope_decl not a global_scope.
	* src/abg-dwarf-reader.cc (read_context::nil_scope_): Add new data
	member.
	(read_context::{global_scope, nil_scope}): Define new member functions.
	(read_context::build_die_parent_maps): Do not build the map if we
	are looking at a C (or asm) translation unit.
	(get_scope_die, get_scope_for_die): If we are looking at a C
	translation unit then do return the global scope.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>