stage2: make hasCodeGenBits() always true for pointers

* LLVM backend: The `alloc` AIR instruction as well as pointer
   constants which point to a 0-bit element type now call a common
   codepath to produce a `*const llvm.Value` which is a non-zero pointer
   with a bogus-but-properly-aligned address.
 * LLVM backend: improve the lowering of optional types.
 * Type: `hasCodeGenBits()` now returns `true` for pointers even when
   it returns `false` for their element types.

Effectively, #6706 is now implemented in stage2 but not stage1.

test/behavior/optional.zig

@@ -36,3 +36,11 @@ test "self-referential struct through a slice of optional" {
     var n = S.Node.new();
     try expect(n.data == null);
 }
+
+pub const EmptyStruct = struct {};
+
+test "optional pointer to size zero struct" {
+    var e = EmptyStruct{};
+    var o: ?*EmptyStruct = &e;
+    try expect(o != null);
+}