Attempt to appease C++11 buildbots

Explicit operator bool doesn't not play nicely with gtest assertion
macros (i.e. it performs as advertised and I wish that gtest subverted
it for me).

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@201639 91177308-0d34-0410-b5e6-96231b3b80d8