[llvm] [MC] Reduce size of MCDataFragment by 8 bytes (PR #95293)

[via llvm-commits]

aengelke wrote:

MCDataFragment goes from 224 to 216 bytes, MCFragment from 39 to 38. MCEncodedFragment adds to bytes and now goes from 41 to 40 bytes. In the C++ Itanium ABI, there is no padding after base classes.

https://github.com/llvm/llvm-project/pull/95293