roll: third_party/pigweed pw_build: Support pw_facade without public headers

- Allow use of pw_facade without public headers, but do not emit a
  .facade subtarget when it is empty. pw_facade still provides value
  since it handles unset backend variables and gives helpful errors.
- Propagate visibility to the ".facade" target. Fix existing visibility
  violations.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/239755
Original-Revision: b6e84ae48f25f37da692df0b5fcc148ffa9d3d58

Rolled-Repo: https://pigweed.googlesource.com/pigweed/pigweed
Rolled-Commits: dc99cbcc9e8026..b6e84ae48f25f3
Roller-URL: https://ci.chromium.org/b/8735049298557974641
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I234842593bb697010328293ab771f9fdb556af44
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/examples/+/240092
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Lint: Lint 🤖 <android-build-ayeaye@system.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 dc99cbc..b6e84ae 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit dc99cbcc9e80267da9a19dfd49fdf4be77848a92
+Subproject commit b6e84ae48f25f37da692df0b5fcc148ffa9d3d58