more clang analogues of gcc warnings we report

git-svn-id: https://svn.r-project.org/R/trunk@66558 00db46b3-68df-0310-9c12-caf00c1e9a41
typos

git-svn-id: https://svn.r-project.org/R/trunk@66559 00db46b3-68df-0310-9c12-caf00c1e9a41
tweak

git-svn-id: https://svn.r-project.org/R/trunk@66560 00db46b3-68df-0310-9c12-caf00c1e9a41