Move LinkageInfo out of NamedDecl so that it can be used in Type.h.

Everything that cares about visibility also cares about linkage, so I just
moved it to Visibility.h instead of creating a new .h.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176155 91177308-0d34-0410-b5e6-96231b3b80d8