roll: third_party/pigweed pw_toolchain: llvm_clang.BUILD -> llvm_clang_legacy.BUILD part 1/2

Copies the toolchain build file to an alternative name as part of a
non-breaking transition to move the file.

Original-Bug: b/346388161
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/229730
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Original-Revision: 461c43a49e425e973644b4a58ebc5a2ab3e86c04

Rolled-Repo: https://pigweed.googlesource.com/pigweed/pigweed
Rolled-Commits: 170ea861755bd7..461c43a49e425e
Roller-URL: https://ci.chromium.org/b/8736591588069638945
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I171adfb92ed952cfb7d619219156ab32bc136468
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/236354
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.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 170ea86..461c43a 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 170ea861755bd7186f3da240ac770729f6a42015
+Subproject commit 461c43a49e425e973644b4a58ebc5a2ab3e86c04