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

[Fangrui Song via llvm-commits]

MaskRay wrote:

For the complex aarch64 Linux kernel https://github.com/ClangBuiltLinux/linux/issues/2116 , 
"MCFragment: Use trailing data for fixed-size part" creates some 0-size Data fragments, which triggers an edge case that is not correctly handled by MCAssembler::layout. Adding an extra `relaxOnce` and `layoutSection` makes the test pass.

I reverted the commit https://github.com/llvm/llvm-project/commit/a2fef664c29a53bfa8a66927fcf8b2e5c9da4876 ("MCFragment: Use trailing data for fixed-size part") as a workaround.

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