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

[Fangrui Song via llvm-commits]

MaskRay wrote:

> The symbol including this member is being overwritten here:
> 
> https://github.com/llvm/llvm-project/blob/2117677e304d334326f6591f3c75fb2f34dc4bcb/lld/COFF/SymbolTable.cpp#L496-L509
> 
> I don't think that's compatible with this `const` qualifier.

I recall this discussion 
https://discourse.llvm.org/t/can-we-add-invariant-group-for-const-class-members/75465 

@zygoloid : Is the `const` use UB? Even if no, should it be discouraged?

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