[ASTMatchers] Add a usingDirectiveDecl matcher.

This matches 'using namespace' declarations.

Differential Revision: http://reviews.llvm.org/D4517

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213152 91177308-0d34-0410-b5e6-96231b3b80d8