[llvm] MC: Clean up code related to fragments (PR #149721)

[Fangrui Song via llvm-commits]

MaskRay wrote:

I'll rename `isVirtualSection` `IsVirtual` separately. The a.out object file format has the BSS concept, which has been
inherited by COFF, XCOFF, Mach-O, and ELF object file formats. LLVM should not create a term "virtual" that is confusing and not used elsewhere.

https://github.com/llvm/llvm-project/pull/149721