[mlir][llvm] Rename void debug type to null

This commit renames the "di_void_result_type" to "di_null_type" as LLVM
does use null not exclusively for void types. An added test demonstrates
this for variadic function declarations, whose DISubroutine indicates the
start of variadic types with `null`.

Reviewed By: gysit

Differential Revision: https://reviews.llvm.org/D144109