Make HasWinCFI a plain bool instead of Optional<bool>
Reid suggested making HasWinCFI a plain bool defaulting to false in D50288.
It's needed in order to add HasWinCFI to MIRPrinter. Otherwise, we'll get the
HasWinCFI.hasValue() && "HasWinCFI not set yet!"'
Reviewers: rnk, mstorsjo, hans, javed.absar
Reviewed By: rnk
Subscribers: kristof.beyls, llvm-commits
Differential Revision: https://reviews.llvm.org/D51560