[roll third_party/pigweed] pw_symbolizer: LlvmSymbolizer tool improvement

This CL changes the LlvmSymbolizer class to better support using this
library in a long lived server process.

- Add an llvm-symbolizer path argument when not in the Pigweed
  environment.
- Add `close` method to handle deterministically closing the
  llvm-symbolizer subprocess.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/168863

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 1972fe82e554003..612a8b64cae1581
Roller-URL: https://ci.chromium.org/b/8771315847187189249
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I8c1bad3558902d8b571ffb735e1c2ad3c734f061
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/169072
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 1972fe8..612a8b6 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 1972fe82e554003a47896b136d63019db951ed53
+Subproject commit 612a8b64cae158121307337512a8f38dfe984bf5