Update RecursiveASTVisitor so that it visits attributes.  This is currently
important for thread safety attributes, which contain expressions that were
not being visited, and were thus invisible to various tools.  There are now
Visit*Attr methods that can be overridden for every attribute.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@198224 91177308-0d34-0410-b5e6-96231b3b80d8