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

[Rahul Joshi via llvm-commits]

jurahul wrote:

:( That does not work. What seems to work is moving `InsnBitWidth` into an anonymous namespace in the generated code and also moving all the specializations into anonymous namespace. Testing that now locally 

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