Implement and test aggregate initialization in C++. Major changes:

  - Support initialization of reference members; complain if any
  reference members are left uninitialized.
  - Use C++ copy-initialization for initializing each element (falls
  back to constraint checking in C)
  - Make sure we diagnose when one tries to provide an initializer
  list for a non-aggregate.
  - Don't complain about empty initializers in C++ (they are permitted)
  - Unrelated but necessary: don't bother trying to convert the
  decl-specifier-seq to a type when we're dealing with a C++
  constructor, destructor, or conversion operator; it results in
  spurious warnings.

llvm-svn: 63431