roll: third_party/pigweed: pw_async2: Use variant in awaitable

This allows for a smaller awaitable object as it is no longer necessary
to store both the Pendable and the output at once.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/233771

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 0acb4afcc63a107..2fbf9d847123b2e
Roller-URL: https://ci.chromium.org/b/8737701209159546977
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I9a33e02ad0e59ce9915a5c15e6a10f25f63f050a
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/233955
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
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 0acb4af..2fbf9d8 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 0acb4afcc63a10701b796bcd271acd25ce100f0a
+Subproject commit 2fbf9d847123b2ed2dfa2e9ff61cec2ba30324fc