ir: Avoid duplicates when reading member functions

ir: Avoid duplicates when reading member functions

As the same class can be read several times, let's avoid having
duplicates of a given virtual member function.

	* src/abg-ir.cc (fixup_virtual_member_function): Check if a
	virtual member function with the same linkage name exists already
	before adding it in.
	* tests/data/test-annotate/test18-pr19037-libvtkRenderingLIC-6.1.so.abi:
	Adjust.
	* tests/data/test-annotate/test20-pr19025-libvtkParallelCore-6.1.so.abi:
	Adjust.
	* tests/data/test-read-dwarf/PR22122-libftdc.so.abi: Adjust.
	* tests/data/test-read-dwarf/test10-pr18818-gcc.so.abi: Adjust.
	* tests/data/test-read-dwarf/test18-pr19037-libvtkRenderingLIC-6.1.so.abi:
	Adjust.
	* tests/data/test-read-dwarf/test20-pr19025-libvtkParallelCore-6.1.so.abi:
	Adjust.
	* tests/data/test-read-dwarf/test22-pr19097-libstdc++.so.6.0.17.so.abi:
	Adjust.
	* tests/data/test-read-dwarf/test9-pr18818-clang.so.abi: Adjust.

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