Always set dso_local in CodeGenModule::setDSOLocal.

This shouldn't change any results for now, but is more consistent with
how we set dllimport/dllexport and will make future changes easier.

Since clang produces IR as it parses, it can find out mid file that
something is dllimport. When that happens we have to drop
dso_local. This is not a problem right now because
CodeGenModule::setDSOLocal is called from relatively few places at
the moment.

llvm-svn: 326527