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

[Fangrui Song via llvm-commits]

MaskRay wrote:

> Ok. What about new Fragment() in `X86/MCTargetDesc/X86AsmBackend.cpp` and `MCPseudoProbe.cpp`? Then all new Fragment() would be gone, I think.

Update `X86/MCTargetDesc/X86AsmBackend.cpp` and `MCPseudoProbe.cpp` and `MCCodeView.cpp`.

Perhaps `MCSection::flushPendingLabels` `new MCDataFragment` is the only remaining one.

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