Fix linkage of static locals in available_externally functions to be DiscardableODR/linkonce_odr

As Mehdi put it, entities should either be
available_externally+weak_odr, or linkonce_odr+linkonce_odr. While some
functions are emitted a_e/weak, their local variables were emitted
a_e/linkonce_odr.

While it might be nice to emit them a_e/weak, the Itanium ABI (& best
guess at MSVC's behavior as well) requires the local to be
linkonce/linkonce.

Reviewers: rsmith, mehdi_amini

Differential Revision: https://reviews.llvm.org/D29233

llvm-svn: 293344