Provide a default constructor for ObjCInterfaceDecl::DefinitionData

that value-initializes all of its members for which
default-initialization does not suffice. This should clean up the
failures for compilers that do not implement C++ DR543:

  http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#543

and, therefore, implement very unfortunate semantics for
value-initialization of classes with implicitly-declared, non-trivial
default constructors.

llvm-svn: 146658