[llvm] MCFragment: Use trailing data for fixed-size part (PR #150846)

[Fangrui Song via llvm-commits]

MaskRay wrote:

While I am still waiting for results on https://llvm-compile-time-tracker.com/ , I think this is in good shape for review (thanks a lot for past reviews and suggestions!)

https://github.com/llvm/llvm-project/pull/150846