roll: third_party/pigweed pw_toolchain: Move fuchsia_clang toolchains to pw_toolchain

Moves the c++ toolchain definitions for fuchsia clang to the pigweed
repository to avoid having to download the fuchsia_clang repository
during toolchain resolution.

Original-BUG: 346416385
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/237915
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Original-Revision: a9f2d91c85fe017e27ec94b7502984818cc7c723

Rolled-Repo: https://pigweed.googlesource.com/pigweed/pigweed
Rolled-Commits: 6422c9ae137ad3..a9f2d91c85fe01
Roller-URL: https://ci.chromium.org/b/8735624636394510689
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I36006bb6e05ec90f895fc8615d0d0dc95677ded9
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/examples/+/238347
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 6422c9a..a9f2d91 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 6422c9ae137ad37a9ec172dcd95f294ce5631f82
+Subproject commit a9f2d91c85fe017e27ec94b7502984818cc7c723