comptime: empty-member types are valid for all kinds; keep never-defined declare rejected

A comptime-constructed type with NO members is now VALID for every kind
(empty struct, empty tuple, empty enum, empty tagged_union) — only a bare
`declare("X")` placeholder that is never completed by a matching `define`
stays rejected (it would panic codegen).

- comptime_vm.zig registerTypeVm: drop the blanket "a type with no members
  is never valid" rejection. The per-kind loops are vacuous for an empty
  member list and the dup-name checks stay correct.
- types.zig TaggedUnionInfo: add `defined: bool = true`. Every real
  construction (normal unions, error sets, register_type completion) is
  "defined" by default; only the two declare-PLACEHOLDER sites set it false:
  comptime_vm.declareNominal and lower/comptime.preregisterForwardTypes.
- lower/comptime.checkComptimeTypeResult: reject on `!defined` (never-defined
  placeholder) instead of `fields.len == 0`, so an explicitly-defined empty
  union passes through while a never-completed declare is still gated.
- types.zig typeSizeBytes(tagged_union): floor the payload area at 8 bytes
  when no field carries a payload, mirroring the LLVM lowering — fixes a
  verifySizes panic on an empty/all-void tagged_union (IR sized to tag-only,
  LLVM laid out tag + [8 x i8]).

Tests:
- examples/1179: repurposed from "empty enum rejected" (now valid) to the
  never-defined `declare` case (the remaining rejection); preserves its
  issue-0140 regression role.
- examples/1180 (duplicate variant): still rejected, unchanged output.
- examples/0641 (new): construct empty struct/tuple/enum/tagged_union via
  define/declare; instantiate the constructible ones; exit 0.

examples/expected/0641-comptime-empty-types-valid.exit

@@ -0,0 +1 @@
+0

examples/expected/0641-comptime-empty-types-valid.stderr

@@ -0,0 +1 @@
+

examples/expected/0641-comptime-empty-types-valid.stdout

@@ -0,0 +1 @@
+empty struct/tuple/enum/tagged_union are all valid

examples/expected/1179-diagnostics-comptime-type-construction-bail.stderr

@@ -1,5 +1,5 @@
-error: comptime type construction failed: comptime register_type: a type with no members is never valid
-  --> examples/1179-diagnostics-comptime-type-construction-bail.sx:14:10
+error: type 'Undefined' is declared but never defined — complete it with define(handle, info)
+  --> examples/1179-diagnostics-comptime-type-construction-bail.sx:25:14
    |
-14 | Empty :: define(declare("Empty"), .enum(.{ variants = .[] }));
-   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+25 | Undefined :: mk_undefined();
+   |              ^^^^^^^^^^^^^^