ExtWarn/Warnings. Mostly the name of the warning was changed to match the
semantics, but in the PR20356 cases, the warning was about valid code, so the
diagnostic was changed from ExtWarn to Warning instead.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213443 91177308-0d34-0410-b5e6-96231b3b80d8