[CodeGen/AccelTable]: Handle -dwarf-linkage-names=Abstract correctly

Summary:
If we are not emitting a linkage name in the .debug_info sections, we
should not add it into the index either. This makes sure our index is
consistent with the actual debug info.

I am also explicitly setting the --dwarf-linkage-names=All in the
name-collsions test as that one would now fail on targets where this
defaults to "Abstract" (in fact, it would have failed already if there
wasn't a bug in the DWARF verifier, which I fix as well).

Reviewers: probinson, aprantl, JDevlieghere

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D46748

llvm-svn: 332246