[MC] Allocate MCFragment with a bump allocator
#95197 and 75006466296ed4b0f845cbbec4bf77c21de43b40 eliminated all raw
`new MCXXXFragment`. We can now place fragments in a bump allocator.
In addition, remove the dead `Kind == FragmentType(~0)` condition.
~CodeViewContext may call `StrTabFragment->destroy()` and need to be
reset before `FragmentAllocator.Reset()`.
Tested by llvm/test/MC/COFF/cv-compiler-info.ll using asan.
Pull Request: https://github.com/llvm/llvm-project/pull/96402
