devcontainer: Add clang, gfortran, and valgrind

Provide `clang` for developers who prefer to build with it rather than
with the default `g++`, and `valgrind` to run CMake and its tests under a
memory checker.  The sanitizer runtimes already come with both
compilers, so building with `-fsanitize=` needs nothing installed.

Name `gfortran` as well, without which Clang cannot link at all.  Ubuntu
26.04 carries a Fortran-only `gcc-16` build that provides no C++ standard
library, `libopenmpi-dev` lists `gfortran-16` first among the
alternatives it accepts, and Clang selects the newest GCC installation it
finds.  Naming `gfortran` lets `apt` satisfy that dependency with
`gfortran-15` instead, so the GCC 16 installation never appears.

Suggested-by: Tyler Yankee <tyler.yankee@kitware.com>

.devcontainer/dev_packages.lst

@@ -17,13 +17,24 @@ nano
 unminimize
 vim
 
-# Build CMake.
+# Build CMake.  `g++` comes from `deps_packages.lst`, as CI builds with it;
+# `clang` is here for developers who prefer to build with it instead.
+#
+# `gfortran` is named only to keep the Fortran-only `gcc-16` build that
+# Ubuntu 26.04 carries out of the container: `libopenmpi-dev` lists
+# `gfortran-16` first among the alternatives it accepts, and Clang, which
+# selects the newest GCC installation it finds, cannot link against one that
+# provides no C++ standard library.
 ccache
+clang
 cmake
+gfortran
 ninja-build
 
-# Debug CMake.
+# Debug and diagnose CMake.  The sanitizer runtimes come with the compilers,
+# so building with `-fsanitize=` needs nothing installed here.
 gdb
+valgrind
 
 # Build the documentation.
 python3-sphinx

Help/dev/devcontainer.rst

@@ -111,6 +111,13 @@ against, the container provides:
   Ubuntu release with ``-rc`` appended.  Add it to the ``Suites`` field of
   ``/etc/apt/sources.list.d/kitware.sources`` to install those as well.
 
+* ``clang``, for developers who would rather build with it than with the
+  default ``g++``:
+
+  .. code-block:: console
+
+    $ cmake -G Ninja -B build-clang -S . -DCMAKE_CXX_COMPILER=clang++
+
 * ``ccache``, to speed up repeated builds, e.g.:
 
   .. code-block:: console
@@ -140,6 +147,18 @@ against, the container provides:
 
 * ``gdb``, to debug CMake as described in the `CMake Debugging Guide`_.
 
+* ``valgrind``, and the sanitizer runtimes that come with ``g++`` and
+  ``clang``, to run CMake and its tests under a memory checker, the way the
+  sanitizer and Valgrind jobs of our CI do:
+
+  .. code-block:: console
+
+    $ cmake -G Ninja -B build-asan -S . \
+        -DCMAKE_C_FLAGS=-fsanitize=address \
+        -DCMAKE_CXX_FLAGS=-fsanitize=address
+    $ cmake --build build-asan
+    $ ctest --test-dir build-asan
+
 * ``glab``, the `GitLab CLI`_, to work with merge requests, issues, and
   pipelines on our GitLab instance, and `glab-axi`_, a wrapper around it
   whose output follows the `AXI`_ conventions: