fix PR3281:accepted0[02].ll: represent empty arrays distinctly, and

diagnose attempts to initialize non-empty arrays with them.  This 
produces:
llvm-as: accepted02.ll:1:28: invalid empty array initializer
@"o" = global [5 x double] []
                           ^
llvm-as: accepted00.ll:1:32: invalid empty array initializer
@"za" = thread_local global {} []
                               ^
[

llvm-svn: 61676