[roll third_party/pigweed] pw_span: Temporary workaround for pw::span migration

Provide both pw::span and std::span from //pw_span to simplify the
migration.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/99544

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: b01aa80b67133ff..c6b37efe0a4e048
Roller-URL: https://ci.chromium.org/b/8810521788643324897
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8810521788643324897
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I8fc636bd338023b140b30209c9c67819904e0135
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/99538
Bot-Commit: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index b01aa80..c6b37ef 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit b01aa80b67133ffa422d6f5fe49a1308eb058910
+Subproject commit c6b37efe0a4e048f8df144a0129ac518580c46bc