Remove another 'using namespace llvm' from the

RecursiveSymbolVisitor.h header

This should fix the modules build
(http://lab.llvm.org:8011/builders/clang-x86_64-linux-selfhost-modules-2/).


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@307908 91177308-0d34-0410-b5e6-96231b3b80d8