They are currently implicit because TargetMachine::shouldAssumeDSOLocal implies
dso_local.
For external data, clang -fno-pic emits the dso_local specifier for ELF and
non-MinGW COFF. Adding explicit dso_local makes these tests in align with the
clang behavior and helps implementing an option to use GOT indirection for
external data access in -fno-pic mode (to avoid copy relocations).
This is a special calling convention to be used by the GHC compiler.
Author: Stefan Schulze Frielinghaus
Differential Revision: https://reviews.llvm.org/D69024
