Refactor protected virtual functions from SymbolFile into new SymbolFileCommon class.

This is a preparatory patch for https://reviews.llvm.org/D121631.
It refactors protected virtual members of SymbolFile
into a new SymbolFileCommon class per suggestion in:
https://reviews.llvm.org/D121631

This will avoid the friendship declaration in that patch.

Differential Revision: https://reviews.llvm.org/D124110