Add a namespace qualifier to the befriending statement for
RecusiveASTVisitor. With Clang and modern GCCs this was found through
the injected class name of the base class but older GCCs don't properly
implement the injected class name rules.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@171593 91177308-0d34-0410-b5e6-96231b3b80d8