Changed to static_cast, due to post-commit review.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173980 91177308-0d34-0410-b5e6-96231b3b80d8
Diagnostics: Clarify name of line-length-limiting constant in r173976.

Thanks, Sean.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173981 91177308-0d34-0410-b5e6-96231b3b80d8