CodeGen: Fix linkage of reference temporaries

Summary:
A reference temporary should inherit the linkage of the variable it
initializes.  Otherwise, we may hit cases where a reference temporary
wouldn't have the same value in all translation units.

Reviewers: rsmith

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D3515

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207451 91177308-0d34-0410-b5e6-96231b3b80d8