Bug 21630 - A this pointer DIE can be const

Bug 21630 - A this pointer DIE can be const

From what I have usually seen emitted by GCC, a "this pointer" is
represented by a DW_TAG_pointer_type.  If the "this pointer" is for a
const method, then it points to a const class type DIE.

But in, in this problem report, I am seeing a "this pointer"
represented by a DW_TAG_const_type that points to a
DW_TAG_pointer_type, which is itself points to a class type DIE.

The code of die_this_pointer_is_const needs to be adjusted so that it
doesn't expect only a DW_TAG_pointer_type.

This is what this patch does.

	* src/abg-dwarf-reader.cc (die_this_pointer_is_const): If the DIE
	is not a DW_TAG_pointer_type then don't crash.

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