[llvm] [MC] Add MCFragment allocation helpers (PR #95197)

[via llvm-commits]

https://github.com/aengelke commented:

Ok, but what about the other places with `new MCXXXFragment`? MCPseudoProbe, MCELFStreamer and MCMachOStreamer have a few insert(new Fragment) calls, which could be trivially replaced by allocAndAdd.

https://github.com/llvm/llvm-project/pull/95197