[llvm] [NFC][MC][Decoder] Extract fixed pieces of decoder code into new header file (PR #154802)

[Rahul Joshi via llvm-commits]

jurahul wrote:

Linux failure seems unrelated:
FAIL: LLDB (/home/gha/actions-runner/_work/llvm-project/llvm-project/build/bin/clang-x86_64) :: test_moduleSymbols (TestDAP_moduleSymbols.TestDAP_moduleSymbols.test_moduleSymbols)

Will rebase and trigger another run

https://github.com/llvm/llvm-project/pull/154802