[lld] Replace Symbolize.h with DIContext.h in lld's COFF lib

lld only needs DIContext.h which it gets through Symbolize.h -> SymbolizableModule.h -> DIContext.h. This replaces it with a direct include of DIContext.h to avoid any confusion and pulling in unnecessary headers.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D115659