ci: add runners for vanilla LLVM 20

Ubuntu 25.04 has `llvm-20` packages that we can start testing with.
The `Dockerfile` is otherwise the same as the `llvm-18`/`19` runners.

Author: cuviper

src/ci/docker/host-x86_64/x86_64-gnu-llvm-19/Dockerfile

@@ -3,6 +3,7 @@ FROM ubuntu:24.10
 ARG DEBIAN_FRONTEND=noninteractive
 
 RUN apt-get update && apt-get install -y --no-install-recommends \
+  bzip2 \
   g++ \
   gcc-multilib \
   make \

src/ci/docker/host-x86_64/x86_64-gnu-llvm-20/Dockerfile

@@ -0,0 +1,69 @@
+FROM ubuntu:25.04
+
+ARG DEBIAN_FRONTEND=noninteractive
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+  bzip2 \
+  g++ \
+  gcc-multilib \
+  make \
+  ninja-build \
+  file \
+  curl \
+  ca-certificates \
+  python3 \
+  git \
+  cmake \
+  sudo \
+  gdb \
+  llvm-20-tools \
+  llvm-20-dev \
+  libedit-dev \
+  libssl-dev \
+  pkg-config \
+  zlib1g-dev \
+  xz-utils \
+  nodejs \
+  mingw-w64 \
+  # libgccjit dependencies
+  flex \
+  libmpfr-dev \
+  libgmp-dev \
+  libmpc3 \
+  libmpc-dev \
+  && rm -rf /var/lib/apt/lists/*
+
+# Install powershell (universal package) so we can test x.ps1 on Linux
+# FIXME: need a "universal" version that supports libicu74, but for now it still works to ignore that dep.
+RUN curl -sL "https://github.com/PowerShell/PowerShell/releases/download/v7.3.1/powershell_7.3.1-1.deb_amd64.deb" > powershell.deb && \
+    dpkg --ignore-depends=libicu72 -i powershell.deb && \
+    rm -f powershell.deb
+
+COPY scripts/sccache.sh /scripts/
+RUN sh /scripts/sccache.sh
+
+# We are disabling CI LLVM since this builder is intentionally using a host
+# LLVM, rather than the typical src/llvm-project LLVM.
+ENV NO_DOWNLOAD_CI_LLVM 1
+ENV EXTERNAL_LLVM 1
+
+# Using llvm-link-shared due to libffi issues -- see #34486
+ENV RUST_CONFIGURE_ARGS \
+      --build=x86_64-unknown-linux-gnu \
+      --llvm-root=/usr/lib/llvm-20 \
+      --enable-llvm-link-shared \
+      --set rust.randomize-layout=true \
+      --set rust.thin-lto-import-instr-limit=10
+
+COPY scripts/shared.sh /scripts/
+
+ARG SCRIPT_ARG
+
+COPY scripts/add_dummy_commit.sh /tmp/
+COPY scripts/x86_64-gnu-llvm.sh /tmp/
+COPY scripts/x86_64-gnu-llvm2.sh /tmp/
+COPY scripts/x86_64-gnu-llvm3.sh /tmp/
+COPY scripts/stage_2_test_set1.sh /tmp/
+COPY scripts/stage_2_test_set2.sh /tmp/
+
+ENV SCRIPT "/tmp/add_dummy_commit.sh && /tmp/${SCRIPT_ARG}"

src/ci/github-actions/jobs.yml

@@ -301,6 +301,31 @@ auto:
   - name: x86_64-gnu-distcheck
     <<: *job-linux-8c
 
+  # The x86_64-gnu-llvm-20 job is split into multiple jobs to run tests in parallel.
+  # x86_64-gnu-llvm-20-1 skips tests that run in x86_64-gnu-llvm-20-{2,3}.
+  - name: x86_64-gnu-llvm-20-1
+    env:
+      RUST_BACKTRACE: 1
+      IMAGE: x86_64-gnu-llvm-20
+      DOCKER_SCRIPT: stage_2_test_set1.sh
+    <<: *job-linux-4c
+
+  # Skip tests that run in x86_64-gnu-llvm-20-{1,3}
+  - name: x86_64-gnu-llvm-20-2
+    env:
+      RUST_BACKTRACE: 1
+      IMAGE: x86_64-gnu-llvm-20
+      DOCKER_SCRIPT: x86_64-gnu-llvm2.sh
+    <<: *job-linux-4c
+
+  # Skip tests that run in x86_64-gnu-llvm-20-{1,2}
+  - name: x86_64-gnu-llvm-20-3
+    env:
+      RUST_BACKTRACE: 1
+      IMAGE: x86_64-gnu-llvm-20
+      DOCKER_SCRIPT: x86_64-gnu-llvm3.sh
+    <<: *job-linux-4c
+
   # The x86_64-gnu-llvm-19 job is split into multiple jobs to run tests in parallel.
   # x86_64-gnu-llvm-19-1 skips tests that run in x86_64-gnu-llvm-19-{2,3}.
   - name: x86_64-gnu-llvm-19-1