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

[Derek Schuff via llvm-commits]

dschuff wrote:

I haven't actually reviewed this PR, but since it popped up in my notifications and it might be relevant to MCFragment refactoring, I wanted to let you know that we've finished the Native Client deprecation in Chromium, and removing NaCl support from LLVM is on my TODO list. So this might mean that we can also remove bundle alignment support from the assembler (assuming there are no users other than NaCl?) and some things could be simplified.

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