Implement semantic analysis for the GNU flexible array initialization

extension. The interaction with designated initializers is a
bit... interesting... but we follow GNU's lead and don't permit too
much crazy code in this area.

Also, make the "excess initializers" error message a bit more
informative.

Addresses PR2561: http://llvm.org/bugs/show_bug.cgi?id=2561

llvm-svn: 63785