Devirtualize and pack MCFragment to reduce memory usage.

MCFragment didn't really need vtables.  The majority of virtual methods were just getters and setters.

This removes the vtables and uses dispatch on the kind to do things like delete which needs to
get the appropriate class.

This reduces memory on the verify use list order test case by about 2MB out of 800MB.

Reviewed by Rafael Espíndola

llvm-svn: 239952