Merge storage classes even when contexts don't match.

This fixes the storage class of extern decls that are merged with file level
statics. The patch also fixes the linkage computation so that they are
considered internal.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@170406 91177308-0d34-0410-b5e6-96231b3b80d8