[llvm-dwarfutil][DWARFv5] fix reading of DW_FORM_addrx attribute.

llvm::dwarfutil::ObjFileAddressMap::relocateIndexedAddr() does not
read address value. The relocateIndexedAddr() should not relocate
the address as the linked binary has already resolved relocations.
But it should read the value. This patch adds the reading value
of the address.

Differential Revision: https://reviews.llvm.org/D133324