Auto merge of #147463 - dianqk:test/llvm21, r=<try>

[Do not merge] Test LLVM 21.1.2

try-job: x86_64-gnu-distcheck

Author: rust-bors[bot]

.gitmodules

@@ -24,8 +24,8 @@
 	shallow = true
 [submodule "src/llvm-project"]
 	path = src/llvm-project
-	url = https://github.com/rust-lang/llvm-project.git
-	branch = rustc/21.1-2025-08-01
+	url = https://github.com/dianqk/llvm-project.git
+	branch = test/llvm21
 	shallow = true
 [submodule "src/doc/embedded-book"]
 	path = src/doc/embedded-book

src/ci/docker/host-x86_64/x86_64-gnu-distcheck/Dockerfile

@@ -1,5 +1,5 @@
 # Runs `distcheck`, which is a collection of smoke tests:
-# 
+#
 # - Run `make check` from an unpacked dist tarball to make sure we can at the
 #   minimum run check steps from those sources.
 # - Check that selected dist components at least have expected directory shape
@@ -10,7 +10,7 @@
 #
 # FIXME(#136822): dist components are generally under-tested.
 
-FROM ubuntu:22.04
+FROM ubuntu:24.04
 
 ARG DEBIAN_FRONTEND=noninteractive
 RUN apt-get update && apt-get install -y --no-install-recommends \