[llvm] Add assertions for the smart pointers with the possibility to be null...

[llvm] Add assertions for the smart pointers with the possibility to be null in DWARFLinker::loadClangModule

Split from D91844.

The local variable `Unit` in function `DWARFLinker::loadClangModule`
in file `llvm/lib/DWARFLinker/DWARFLinker.cpp`. If the variable is not set
in the loop below its definition, it will trigger a null pointer dereference
after the loop.

Patch By: OikawaKirie

Reviewed By: avl

Differential Revision: https://reviews.llvm.org/D97185