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

[Fangrui Song via llvm-commits]

MaskRay wrote:

> LGTM. Re flushPendingLabels -- maybe for now pass a context as parameter so that the new() is gone there, too? We then could switch to MCContext's allocator.

Thanks. Let me change `flushPendingLabels` separately...

I guess a bump allocator helps glibc malloc but less for mimalloc.

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