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

[Jason Eckhardt via llvm-commits]

nvjle 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.

FWIW, you can subscribe to https://github.com/orgs/llvm/teams/pr-subscribers-mc to get notifications for patches to the MC layer. (we're also feeling the churn downstream).

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