[sanitizer_common] Allow customization of StartSymbolizerSubprocess

Summary:
Make SymbolizerProcess::StartSymbolizerSubprocess virtual and protected
to allow subclasses to customize it via "override and call".

Subscribers: kubamracek, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D65252

llvm-svn: 366967