Fix map key in header-abi-diff while extracting user defined types.

The key being added to respective maps which facilitate diffing of
unreferenced user defined types was earlier the linker set key. The
unique id should be used instead. This is because linker set keys for C
and C++ types can be the same, but their unique id's cannot. Thus,
having linker set keys as a key leaves open the possibility of false
type aliasing, whereas the unique id does not.

Test: python3 tests/test.py

Test: python3 tests/test.py \
      MyTest.test_libc_and_cpp_with_unused_struct_and_libc_and_cpp_with_unused_cstruct;
      before this change, this test fails; after this change, the test
      passes.

Change-Id: I91816167a48308a8c812d01e0a09e9bb70c60b50