Allow dllimport/dllexport on inline functions and adjust the linkage.

This is a step towards handling these attributes on classes (PR11170).

Differential Revision: http://reviews.llvm.org/D3772

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208925 91177308-0d34-0410-b5e6-96231b3b80d8