[llvm-branch-commits] [NFCI][ELF] Merge AddendOnly and AddendOnlyWithTargetVA (PR #150797)

[Jessica Clarke via llvm-branch-commits]

https://github.com/jrtc27 created https://github.com/llvm/llvm-project/pull/150797

The former is just a special case of the latter, ignoring the expr and
always just using the addend, allowing (and enforcing) the sym is null.
If we just use dummySym then we don't need to maintain this as a
separate case, since R_ADDEND will return the addend unmodified for the
call to getRelocTargetVA.