[setup] Restructure Ubuntu dependencies on clang-9 (#16622)

On Ubuntu, move clang-9 into a separate "--with-clang" designation,
which is enabled by default to maintain our current behavior. In the
future, we should probably switch it off by default, to save download
time and disk space during builds.

Note that we still unconditionally require Clang's parsing library
and clang-format binary to be installed during a source build,
because we use those during the pydrake build.

Add libomp-9-dev alongside clang-9. That allows us to use -fopenmp;
otherwise, we would have linker errors.

Remove vestigial and broken --without-doc-only option.

setup/BUILD.bazel

@@ -19,10 +19,12 @@ exports_files([
     "ubuntu/binary_distribution/packages-bionic.txt",
     "ubuntu/binary_distribution/packages-focal.txt",
     "ubuntu/source_distribution/packages-bionic.txt",
+    "ubuntu/source_distribution/packages-bionic-clang.txt",
     "ubuntu/source_distribution/packages-bionic-doc-only.txt",
     "ubuntu/source_distribution/packages-bionic-maintainer-only.txt",
     "ubuntu/source_distribution/packages-bionic-test-only.txt",
     "ubuntu/source_distribution/packages-focal.txt",
+    "ubuntu/source_distribution/packages-focal-clang.txt",
     "ubuntu/source_distribution/packages-focal-doc-only.txt",
     "ubuntu/source_distribution/packages-focal-maintainer-only.txt",
     "ubuntu/source_distribution/packages-focal-test-only.txt",

setup/ubuntu/install_prereqs.sh

@@ -31,16 +31,21 @@ while [ "${1:-}" != "" ]; do
     --with-kcov)
       source_distribution_args+=(--with-kcov)
       ;;
+    # Install prerequisites that are only needed to when CC=clang-9, i.e.,
+    # opts-in to the ability to compile Drake's C++ code using Clang.
+    --with-clang)
+      source_distribution_args+=(--with-clang)
+      ;;
+    # Do NOT install prerequisites that are only needed to when CC=clang-9,
+    # i.e., opts-out of the ability to compile Drake's C++ code using Clang.
+    --without-clang)
+      source_distribution_args+=(--without-clang)
+      ;;
     # Install prerequisites that are only needed to run select maintainer
     # scripts. Most developers will not need to install these dependencies.
     --with-maintainer-only)
       source_distribution_args+=(--with-maintainer-only)
       ;;
-    # Do NOT install prerequisites that are only needed to build documentation,
-    # i.e., those prerequisites that are dependencies of bazel run //doc:build.
-    --without-doc-only)
-      source_distribution_args+=(--without-doc-only)
-      ;;
     # Do NOT install prerequisites that are only needed to build and/or run
     # unit tests, i.e., those prerequisites that are not dependencies of
     # bazel { build, run } //:install.

setup/ubuntu/source_distribution/install_prereqs.sh

@@ -11,10 +11,13 @@ set -euo pipefail
 with_doc_only=0
 with_kcov=0
 with_maintainer_only=0
+with_clang=1
 with_test_only=1
 with_update=1
 with_asking=1
 
+# TODO(jwnimmer-tri) Eventually we should default to with_clang=0.
+
 while [ "${1:-}" != "" ]; do
   case "$1" in
     # Install prerequisites that are only needed to build documentation,
@@ -29,6 +32,16 @@ while [ "${1:-}" != "" ]; do
     --with-kcov)
       with_kcov=1
       ;;
+    # Install prerequisites that are only needed to when CC=clang-9, i.e.,
+    # opts-in to the ability to compile Drake's C++ code using Clang.
+    --with-clang)
+      with_clang=1
+      ;;
+    # Do NOT install prerequisites that are only needed to when CC=clang-9,
+    # i.e., opts-out of the ability to compile Drake's C++ code using Clang.
+    --without-clang)
+      with_clang=0
+      ;;
     # Install prerequisites that are only needed to run select maintainer
     # scripts. Most developers will not need to install these dependencies.
     --with-maintainer-only)
@@ -124,6 +137,11 @@ if [[ "${with_doc_only}" -eq 1 ]]; then
   apt-get install ${maybe_yes} --no-install-recommends ${packages}
 fi
 
+if [[ "${with_clang}" -eq 1 ]]; then
+  packages=$(cat "${BASH_SOURCE%/*}/packages-${codename}-clang.txt")
+  apt-get install ${maybe_yes} --no-install-recommends ${packages}
+fi
+
 if [[ "${with_test_only}" -eq 1 ]]; then
   packages=$(cat "${BASH_SOURCE%/*}/packages-${codename}-test-only.txt")
   # Suppress Python 3.8 warnings when installing python3-pandas on Focal.

setup/ubuntu/source_distribution/packages-bionic-clang.txt

@@ -0,0 +1,2 @@
+clang-9
+libomp-9-dev

setup/ubuntu/source_distribution/packages-bionic.txt

@@ -1,4 +1,3 @@
-clang-9
 clang-format-9
 coinor-libclp-dev
 coinor-libcoinutils-dev

setup/ubuntu/source_distribution/packages-focal-clang.txt

@@ -0,0 +1,2 @@
+clang-9
+libomp-9-dev

setup/ubuntu/source_distribution/packages-focal.txt

@@ -1,4 +1,3 @@
-clang-9
 clang-format-9
 coinor-libclp-dev
 coinor-libcoinutils-dev