[llvm] [MC][DecoderEmitter] Fix build warning: explicit specialization cannot have a storage class (PR #156375)

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

@jurahul Could you rebase this PR?  I think it's the right thing to hide these constants in anonymous namespaces, but I just want to build this myself.  Thanks!

https://github.com/llvm/llvm-project/pull/156375