[lld] [lld] Remove const qualifier on symbolKind (NFC) (PR #94753)

[Reid Kleckner via llvm-commits]

rnk wrote:

This seems fine, but some of the subclasses still have vtables, so we're still effectively doing some kind of placement new operation here without updating the other existing Symbol references, and I think that is technically UB without more laundering operations.

https://github.com/llvm/llvm-project/pull/94753