Added warning on structures/unions that are empty or contain only
bit fields of zero size. Warnings are generated in C++ mode and if
only such type is defined inside extern "C" block.
The patch fixed PR5065.

Differential Revision: http://llvm-reviews.chandlerc.com/D2151


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194653 91177308-0d34-0410-b5e6-96231b3b80d8