Add matcher for AccessSpecDecls.

Also, add matchers isPrivate(), isProtected() and isPublic(), that
restrict the matching of such AccessSpecDecls and all other Decls.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176017 91177308-0d34-0410-b5e6-96231b3b80d8