[dwarf reader] Support reference types without explicit DW_AT_byte_size

On x86_64 at least, in the debug info emitted by Clang, reference
types don't necessarily have the DW_AT_byte_size property.  In that
case, assume the size of the pointer type is the address size of the
current translation unit, rather than giving up and not building the
type.

	* src/abg-dwarf-reader.cc (build_reference_type): If the type DIE
	has no DW_AT_byte_size, assume the type size is the translation
	unit's address size.
	* tests/data/test-read-dwarf/test9-pr18818-clang.so.abi: Adjust.
	* tests/data/test-read-dwarf/test12-pr18844.so.abi: Adjust.

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

src/abg-dwarf-reader.cc

@@ -7052,10 +7052,16 @@ build_reference_type(read_context&	ctxt,
   type_base_sptr utype = is_type(utype_decl);
   assert(utype);
 
-  size_t size;
-  if (!die_unsigned_constant_attribute(die, DW_AT_byte_size, size))
-    return result;
-  size *= 8;
+  // if the DIE for the reference type doesn't have a byte_size
+  // attribute then we assume the size of the reference is the address
+  // size of the current translation unit.
+  size_t size = ctxt.cur_tu()->get_address_size();
+  if (die_unsigned_constant_attribute(die, DW_AT_byte_size, size))
+    size *= 8;
+
+  // And the size of the pointer must be the same as the address size
+  // of the current translation unit.
+  assert((size_t) ctxt.cur_tu()->get_address_size() == size);
 
   bool is_lvalue = (tag == DW_TAG_reference_type) ? true : false;