Implement libclang support for using directives (cursor + visitation +

Implement libclang support for using directives (cursor + visitation +
suppressing USRs). Also, fix up the source location information for
using directives so that the declaration location refers to the
namespace name.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@112693 91177308-0d34-0410-b5e6-96231b3b80d8