[roll] Roll fuchsia [fidl][llcpp] Introduce client test utility header

And adds a |ClientChecker| type that is a friend of the client types, so
we get rid of all the ugly direct |operator->()| invocations.

Test: fx test -vo //src/lib/fidl/llcpp
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/582603
Original-Revision: de870934887c05ce8ef57cd0b57f413c91e7c405
GitOrigin-RevId: c541d44a91706198ff4e18a9c34bd038c474ac5b
Change-Id: I45183e7b083a59b4652bd8f4ecf33e1a5954c0d2
diff --git a/stem b/stem
index 479cec3..dcdb2a7 100644
--- a/stem
+++ b/stem
@@ -10,7 +10,7 @@
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              remote="https://fuchsia.googlesource.com/fuchsia"
              gerrithost="https://fuchsia-review.googlesource.com"
-             revision="5af7a0681691b352c2e6fa572d798504be7d4582"/>
+             revision="de870934887c05ce8ef57cd0b57f413c91e7c405"/>
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