[yaml2obj][obj2yaml] - Simplify format of the SHT_LLVM_ADDRSIG section.

Previously the description allowed to describe symbols with use of
`Name` and `Index` keys. This patch removes them and now it is still
possible to use either names or symbol indexes, but the code is simpler
and the format is slightly different.

Such a change will be useful for another patches, e.g:
https://reviews.llvm.org/D73788#inline-671077

Differential revision: https://reviews.llvm.org/D73888