[llvm] MC: Restructure MCFragment as a fixed part and a variable tail (PR #148544)

[Steven Wu via llvm-commits]

cachemeifyoucan wrote:

@MaskRay Can you please add me and @rastogishubham as non-blocking reviewer as future changes in MC layer? We have a downstream MCCAS implementation that highly depends on how MC layer is structured. 

https://github.com/llvm/llvm-project/pull/148544