Gracefully (and correctly) handle init of multiple union members

We now emit warnings when doing so and code generation is consistent
with GCC. Note that the C99 spec is unclear as to the precise
behavior.

See also ...
Bug:
  http://llvm.org/bugs/show_bug.cgi?id=16644 and

cfe-dev discussion:
  http://lists.cs.uiuc.edu/pipermail/cfe-dev/2013-September/031918.html

llvm-svn: 191890