[analyzer] Re-enable test I accidentally committed commented-out.

Thanks, Alex!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205720 91177308-0d34-0410-b5e6-96231b3b80d8
[analyzer] Fix mistake in example for potential checker "posix.Errno".

Found by Richard Osbourne!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205721 91177308-0d34-0410-b5e6-96231b3b80d8