Roll third_party/libc++abi/src/ 4a1b48642..a6c815c69 (4 commits)

https://chromium.googlesource.com/external/github.com/llvm/llvm-project/libcxxabi.git/+log/4a1b4864252e..a6c815c69d55

Created with:
  roll-dep third_party/libc++abi/src

Change-Id: I111555e0840340755a9d4650da303fb7933f895d
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/134751
Commit-Queue: Lei Zhang <thestig@chromium.org>
Auto-Submit: Lei Zhang <thestig@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
diff --git a/DEPS b/DEPS
index 341d931..77c2566 100644
--- a/DEPS
+++ b/DEPS
@@ -164,7 +164,7 @@
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling libc++abi
   # and whatever else without interference from each other.
-  'libcxxabi_revision': '4a1b4864252ea043cb995be635c52a0888131308',
+  'libcxxabi_revision': 'a6c815c69d55ec59d020abde636754d120b402ad',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling libpng
   # and whatever else without interference from each other.