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

[Rahul Joshi via llvm-commits]

jurahul 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!

Done

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