Additionally:

 * Sema: fix array cat/mul not setting the sentinel value
   - This required an LLVM backend enhancement to the handling of the
     AIR instruction aggregate_init that likely needs to be
     propagated to the other backends.
 * Sema: report integer overflow of array concatenation in a proper
   compile error instead of crashing.
 * Sema: fix not using proper pointer address space for array cat/mul