Made all other "operator bool"s explicit and ensured
that all clients use them explicitly.  This will hopefully
prevent any future confusion where things get cast to types
we don't expect.

<rdar://problem/15146458>


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@191984 91177308-0d34-0410-b5e6-96231b3b80d8