PR25058 - Better support fn DIEs referring to symbols using DW_AT_ranges

PR25058 - Better support fn DIEs referring to symbols using DW_AT_ranges

In the previous commit 2f7248f, we were just taking the first address
referred to by the DW_AT_ranges attribute as being the address of the
symbol of the function we are looking at.  But there can be cases
where this is not true, as explained at
https://sourceware.org/bugzilla/show_bug.cgi?id=25058#c7.

We really need to get the first address that represents an exported
and defined function symbol, which is pointed to by the DW_AT_ranges
attribute.

And this is what this patch does.

	* src/abg-dwarf-reader.cc
	(read_context::get_first_exported_fn_address_from_DW_AT_ranges):
	Rename read_context::get_first_address_from_DW_AT_ranges into
	this.  Walk through the addresses referred to by the DW_AT_ranges
	attribute until we find one that is for an exported function
	symbol, rather than just picking the first address of the set.
	(read_context::get_function_address): Adjust.

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