clang/lib/CodeGen/CodeGenModule sets dso_local on applicable global variables,
we don't need to duplicate the work in TargetMachine:shouldAssumeDSOLocal.
(Actually the long-term goal (started by r324535) is to remove as much
additional implied dso_local in TargetMachine:shouldAssumeDSOLocal as possible.)
By not implying dso_local, we will respect dso_local/dso_preemptable specifiers
set by the frontend. This allows the proposed -fno-direct-access-external-data
option to work with -fno-pic and prevent copy relocations.
This patch should be NFC in terms of the Clang behavior because the case we
don't set dso_local is a case Clang sets dso_local. However, some tests don't
set dso_local on some `external global` and expose some differences. Most tests
have been fixed to be more robust in previous commits.
