Initialize WarnOnSpellCheck.

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

Issue noticed by Enea Zaffanella.

llvm-svn: 181021

clang/lib/Basic/Diagnostic.cpp

@@ -48,6 +48,7 @@ DiagnosticsEngine::DiagnosticsEngine(
   AllExtensionsSilenced = 0;
   IgnoreAllWarnings = false;
   WarningsAsErrors = false;
+  WarnOnSpellCheck = false;
   EnableAllWarnings = false;
   ErrorsAsFatal = false;
   SuppressSystemWarnings = false;