[roll third_party/pigweed] pw_fuzzer: Move `Domain` from fuzztest::internal to fuzztest

This CL moves the dummy definition of `Domain<T>` to match the real
location in FuzzTest. This allows fuzzer helpers to specify a return
type instead of using `auto`.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/178213

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 666d7adedf73ad8..4e8c2e22d497e72
Roller-URL: https://ci.chromium.org/b/8764971631696732865
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: If6885683036bf1e38642cf2977d5f0e495b8c8e3
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/179851
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 666d7ad..4e8c2e2 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 666d7adedf73ad84d066f615cb8ab8e3b493649f
+Subproject commit 4e8c2e22d497e72c625f12c64633a1d4b963fc78