Make HasWinCFI a plain bool instead of Optional<bool>

Summary:
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
assertion:

HasWinCFI.hasValue() && "HasWinCFI not set yet!"'

Also, a few ARM64 Windows test cases will fail with the same assert if the ARM64
MCLayer part of EH work (D50166) goes in before the frame lowering part that
sets HasWinCFI (D50288 as of now).

Reviewers: rnk, mstorsjo, hans, javed.absar

Reviewed By: rnk

Subscribers: kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D51560

llvm-svn: 341270