Use a bool instead of a bitfield in llvm/ADT/Optional.

Fixes Valgrind failures and removes bitwise operations that don't provide any benefit.
Valgrind failures reported by NAKAMURA Takumi.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171413 91177308-0d34-0410-b5e6-96231b3b80d8