[roll third_party/pigweed] pw_preprocessor: Use __VA_OPT__ when available

When available, use __VA_OPT__ to implement PW_EMPTY_ARGS() instead of
the current workaround. This dramatically simplifies PW_EMPTY_ARGS(),
resulting in:

- code that is easier to understand and debug,
- simpler error messages involving PW_EMPTY_ARGS(),
- support for any number of arguments (currently, PW_EMPTY_ARGS() fails
  to compile for >64 arguments),
- potentially faster compilation times, since PW_EMPTY_ARGS() is
  expanded many, many times in a build (10+ times per tokenized log).

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/187099

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 68f37cc76f34908..8e87382862fa9df
Roller-URL: https://ci.chromium.org/b/8755536617074941761
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: If18cf224f1de95532c66653f69499215b0e8f68d
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/193313
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 68f37cc..8e87382 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 68f37cc76f34908872332986e8d738018d69d416
+Subproject commit 8e87382862fa9df2f0d7de20af3e1c7acbcb236f