Emit an error if an array is too large. We're slightly more strict

than GCC 4.2 here when building 32-bit (where GCC will allow
allocation of an array for which we can't get a valid past-the-end
pointer), and emulate its odd behavior in 64-bit where it only allows
63 bits worth of storage in the array. The former is a correctness
issue; the latter is harmless in practice (you wouldn't be able to use
such an array anyway) and helps us pass a GCC DejaGNU test.

Fixes <rdar://problem/8212293>.

llvm-svn: 111338