Prepare for merging from rust-lang/rust

This updates the rust-version file to caccb4d0368bd918ef6668af8e13834d07040417.
diff --git a/rust-version b/rust-version
index 02b217f..8854fb9 100644
--- a/rust-version
+++ b/rust-version
@@ -1 +1 @@
-21a19c297d4f5a03501d92ca251bd7a17073c08a
+caccb4d0368bd918ef6668af8e13834d07040417