Add LLVM Flang CI

.github/workflows/dependencies/dependencies_flang.sh

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+#!/usr/bin/env bash
+#
+# Copyright 2020-2026 The AMReX Community
+#
+# License: BSD-3-Clause-LBNL
+
+set -eu -o pipefail
+
+# `man apt.conf`:
+#   Number of retries to perform. If this is non-zero APT will retry
+#   failed files the given number of times.
+echo 'Acquire::Retries "3";' | sudo tee /etc/apt/apt.conf.d/80-retries
+
+# Install a recent LLVM Flang (and the matching Clang) from apt.llvm.org.
+# The desired LLVM major version is passed as the first argument.
+if [[ ! -f /etc/apt/trusted.gpg.d/apt.llvm.org.asc ]]; then
+    wget -qO- https://apt.llvm.org/llvm-snapshot.gpg.key | sudo tee /etc/apt/trusted.gpg.d/apt.llvm.org.asc
+fi
+
+source /etc/os-release # set UBUNTU_CODENAME
+
+sudo add-apt-repository "deb http://apt.llvm.org/${UBUNTU_CODENAME}/ llvm-toolchain-${UBUNTU_CODENAME} main"
+sudo add-apt-repository "deb http://apt.llvm.org/${UBUNTU_CODENAME}/ llvm-toolchain-${UBUNTU_CODENAME}-$1 main"
+
+sudo apt-get update
+
+sudo apt-get install -y --no-install-recommends \
+    build-essential      \
+    libfftw3-dev         \
+    clang-$1             \
+    flang-$1             \
+    libomp-$1-dev

.github/workflows/flang.yml

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+name: LinuxFlang
+
+on:
+  push:
+    branches-ignore:
+      - 'dependabot/**'
+  pull_request:
+
+concurrency:
+  group: ${{ github.ref }}-${{ github.head_ref }}-linux-flang
+  cancel-in-progress: true
+
+jobs:
+  check_changes:
+    uses: ./.github/workflows/check_changes.yml
+
+  # Build libamrex and its Fortran interfaces with LLVM Flang.
+  # This guards against LLVM-Flang-specific Fortran errors such as #5603.
+  library_flang:
+    name: Flang@20 DP NOMPI Fortran Interfaces [lib]
+    runs-on: ubuntu-24.04
+    needs: check_changes
+    if: needs.check_changes.outputs.has_non_docs_changes == 'true'
+    steps:
+    - uses: actions/checkout@v7
+    - name: Dependencies
+      run: .github/workflows/dependencies/dependencies_flang.sh 20
+    - name: Build & Install
+      run: |
+        # The apt.llvm.org packages ship the driver as either flang-<ver>
+        # or flang-new-<ver> depending on the LLVM release.
+        FLANG=$(which flang-20 || which flang-new-20)
+
+        mkdir build
+        cd build
+        cmake ..                                       \
+            -DCMAKE_BUILD_TYPE=Debug                   \
+            -DCMAKE_VERBOSE_MAKEFILE=ON                \
+            -DCMAKE_INSTALL_PREFIX=/tmp/my-amrex       \
+            -DAMReX_EB=ON                              \
+            -DAMReX_FORTRAN=ON                         \
+            -DAMReX_FORTRAN_INTERFACES=ON              \
+            -DAMReX_MPI=OFF                            \
+            -DCMAKE_C_COMPILER=$(which clang-20)       \
+            -DCMAKE_CXX_COMPILER=$(which clang++-20)   \
+            -DCMAKE_Fortran_COMPILER=${FLANG}
+        make -j 4
+        make install
+        make test_install