Prepare for merging from rust-lang/rust

This updates the rust-version file to a1dbb443527bd126452875eb5d5860c1d001d761.
diff --git a/rust-version b/rust-version
index a399b5c..f412399 100644
--- a/rust-version
+++ b/rust-version
@@ -1 +1 @@
-425a9c0a0e365c0b8c6cfd00c2ded83a73bed9a0
+a1dbb443527bd126452875eb5d5860c1d001d761