Fix undefined behavior that is caused by not always initializing a bool.

The fix in r310994 is incomplete, as moveFromAndCancel can set the
pointer without initializing OldIsSpeculativelyEvaluating.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@311070 91177308-0d34-0410-b5e6-96231b3b80d8