[roll third_party/pigweed] pw_containers: Make Pair in FlatMap support CTAD

Adding an explicit deduction guide for Pair does a few things:
* Some cases are not covered by implicit deduction guides, for the same
  reason as for std::pair. For more information, see
  https://en.cppreference.com/w/cpp/utility/pair/deduction_guides
* CTAD can be sketchy in some cases, and so some projects might opt to
  enable something like -Wctad-maybe-unsupported. Adding an explicit
  deduction guide is a signal to the compiler that the type is "opting
  in" to CTAD.

Unfortunately, the aggregate initialization with no types does not work
in all circumstances (for example, it can be tricky to get all the
deductions to line up when using std::apply and a lambda), and the CTAD
option exists as a less verbose option than one where the template
parameters _also_ need to be stated explicitly. Note that the Google
style guide doesn't love CTAD in general:
https://google.github.io/styleguide/cppguide.html#CTAD

(Note: CTAD = class template argument deduction)

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/120731

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: f0ea55a197a61f4..878f2906ee814b4
Roller-URL: https://ci.chromium.org/b/8796752217438332369
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I87e06a546c087498f898fce2d055198d2a66bcaf
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/120871
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 f0ea55a..878f290 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit f0ea55a197a61f40ef9c774e4dd0ea06a222f0a0
+Subproject commit 878f2906ee814b44fc314050b81bcb49fa8a0ad7