Classes can be defined in multiple translation units.  This means that
the static constexpr data members should have identical initializers in
all translation units.  Implement this by giving the reference temporary
linkonce_odr linkage.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@229900 91177308-0d34-0410-b5e6-96231b3b80d8