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

[Fangrui Song via llvm-commits]

MaskRay wrote:

![fragment](https://github.com/user-attachments/assets/71f9ffb8-b410-4a8a-9a21-0f7f2a600113)

The `uint32_t FixedSize` member saving does not lead to sizeof(MCFragment) saving due to a 4-byte padding after `VarFixupEnd`

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