[JITLink] Add anonymous symbols in LinkGraph for unnamed temporary symbols

In RISCV, temporary symbols will be used to generate dwarf, eh_frame sections..., and will be placed in object code's symbol table. However, LLVM does not use names on these temporary symbols. This patch add anonymous symbols in LinkGraph for these temporary symbols.

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D116475