Avoid implicit conversions of Optional<T> to bool.

This is a precursor to making Optional<T>'s operator bool 'explicit' when
building Clang & LLVM as C++11.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175722 91177308-0d34-0410-b5e6-96231b3b80d8