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

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 {} []
                               ^
[

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@61676 91177308-0d34-0410-b5e6-96231b3b80d8