[lld] [llvm] Reapply "[RISCV] Support RISCV Atomics ABI attributes (#84597)" (PR #90266)

[Paul Kirth via llvm-commits]

================
@@ -32,8 +32,21 @@ enum AttrType : unsigned {
   PRIV_SPEC = 8,
   PRIV_SPEC_MINOR = 10,
   PRIV_SPEC_REVISION = 12,
+  ATOMIC_ABI = 14,
 };
 
+namespace RISCVAtomicAbiTag {
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ilovepi wrote:

I've followed your advice here, but this seems to require quite a lot of `static_casting` when what we actually want is a typed alias for an integer which is precisely what a standard enum type is. I've also had to introduce what I'd say are useless cases to the switch for unreachable types we've already checked.

https://github.com/llvm/llvm-project/pull/90266