Add DW_AT_GNU_odr_signature to the set of dwarf attributes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186296 91177308-0d34-0410-b5e6-96231b3b80d8
Clarify comments.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186297 91177308-0d34-0410-b5e6-96231b3b80d8