Make Optional<T>'s operator bool 'explicit' in C++11

Provides a general way to add 'explicit' for conversion operators (a no-op when
compiling as C++98).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175723 91177308-0d34-0410-b5e6-96231b3b80d8