Add an explicit operator bool conversion to DIDescriptor to make

Add an explicit operator bool conversion to DIDescriptor to make
it clear what we want to do. Unfortunately the conversion to
pointer operator fires now instead and chasing down all of the
conversions and making them explicit and handled is a large task
so add a FIXME with it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186543 91177308-0d34-0410-b5e6-96231b3b80d8