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/basic.zig

@@ -246,7 +246,7 @@ fn testTakeAddressOfParameter(f: f32) !void {
 }
 
 test "pointer to void return type" {
-    testPointerToVoidReturnType() catch unreachable;
+    try testPointerToVoidReturnType();
 }
 fn testPointerToVoidReturnType() anyerror!void {
     const a = testPointerToVoidReturnType2();