TargetMachine::shouldAssumeDSOLocal currently implies dso_local for such definitions.
Adding explicit dso_local makes these tests align with the clang -fpic behavior
and allow the removal of the TargetMachine::shouldAssumeDSOLocal special case.
Rewrite preemption.ll to dsolocal-static.ll and dsolocal-pic.ll, and add
"PIC Level" metadata.
Fangrui Song