[ThinLTO] Don't internalize weak writeable variables

[ThinLTO] Don't internalize weak writeable variables

Variables with linkonce_odr and weak_odr linkage shouldn't be internalized
if they're not readonly. Otherwise we may end up with multiple copies of
such variable, so reads and writes will become inconsistent

Differential revision: https://reviews.llvm.org/D61255

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@360577 91177308-0d34-0410-b5e6-96231b3b80d8