[PATCH] CodeGen: Emit globals with a constant initializer but no definition as available_externally.

[Richard Smith]

  This seems to be a pre-existing and essentially unrelated bug. The same problem occurs for:

      constexpr int &p = S::q.r;

  I think we should be looking at the linkage of the extending declaration when picking the linkage for a lifetime-extended temporary. (If the extending declaration is externally visible, I think the temporary should be `linkonce_odr`.)

http://llvm-reviews.chandlerc.com/D1982