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

[Alexis Engelke via llvm-commits]

https://github.com/aengelke commented:

Direction looks good, will review more closely tomorrow. However, could you elaborate on why the fixed+variable part are not stored together? This seems to complicate things when accessing them. If growth during relaxation is a concern, we could allocate some padding afterwards to reduce the likelihood of moving the entire contents.

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