[roll third_party/pigweed] pw_span: Remove pw_polyfill dependency

This dependency was added back when the type needed to support more
limited constexpr semantics for C++11 and/or C++14. Those language
versions are no longer targeted, and so the header is now just hanging
around as a ghost.

Note that compatibility_test.cc still depends on pw_polyfill. As a
result, I've added the relevant dependency for the module that builds
that test.

Also, pw_varint/varint.h was naughty and depended on pw_polyfill
transitively through pw_span, so I also have had to fix it.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/124873

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: a81d3692b6bf4ab..782bab04def6c7b
Roller-URL: https://ci.chromium.org/b/8791562131682838225
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I823a35f996dbb71bec5b4f201c790c5d07ec7482
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/126732
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Bot-Commit: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index a81d369..782bab0 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit a81d3692b6bf4ab787b9559d8d3a657881cdc142
+Subproject commit 782bab04def6c7b3c90460b1e6d878b337a5da03