Initialize WarnOnSpellCheck.

Clang always calls setWarnOnSpellCheck, but we shouldn't require every client to
do so.

Issue noticed by Enea Zaffanella.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181021 91177308-0d34-0410-b5e6-96231b3b80d8