[ci] skip clang installation and separate configuration for runtimes.

They will be built with the just built clang either way.
This avoids building the runtimes twice when LLDB is also tested.

.ci/monolithic-linux.sh

diff --git a/.ci/monolithic-linux.sh b/.ci/monolithic-linux.sh
index a56a8bc..694629c 100755
--- a/.ci/monolithic-linux.sh
+++ b/.ci/monolithic-linux.sh
@@ -18,7 +18,6 @@
 
 MONOREPO_ROOT="${MONOREPO_ROOT:="$(git rev-parse --show-toplevel)"}"
 BUILD_DIR="${BUILD_DIR:=${MONOREPO_ROOT}/build}"
-INSTALL_DIR="${BUILD_DIR}/install"
 rm -rf "${BUILD_DIR}"
 
 ccache --zero-stats
@@ -85,69 +84,45 @@
       -D LIBCXX_CXX_ABI=libcxxabi \
       -D MLIR_ENABLE_BINDINGS_PYTHON=ON \
       -D LLDB_ENABLE_PYTHON=ON \
-      -D LLDB_ENFORCE_STRICT_TEST_REQUIREMENTS=ON \
-      -D CMAKE_INSTALL_PREFIX="${INSTALL_DIR}"
+      -D LLDB_ENFORCE_STRICT_TEST_REQUIREMENTS=ON
 
 echo "--- ninja"
+
 # Targets are not escaped as they are passed as separate arguments.
 ninja -C "${BUILD_DIR}" -k 0 ${targets}
 
 runtimes="${3}"
 runtime_targets="${4}"
 
-# Compiling runtimes with just-built Clang and running their tests
-# as an additional testing for Clang.
+# Run runtimes tests.
+# We don't need to do a clean separate build of runtimes, because runtimes
+# will be built against just built clang, and because LIBCXX_TEST_PARAMS
+# and LIBCXXABI_TEST_PARAMS only affect lit configuration, which successfully
+# propagates without a clean build. Other that those two variables, builds
+# are supposed to be the same.
 if [[ "${runtimes}" != "" ]]; then
   if [[ "${runtime_targets}" == "" ]]; then
     echo "Runtimes to build are specified, but targets are not."
     exit 1
   fi
 
-  echo "--- ninja install-clang"
-
-  ninja -C ${BUILD_DIR} install-clang install-clang-resource-headers
-
-  RUNTIMES_BUILD_DIR="${MONOREPO_ROOT}/build-runtimes"
-  INSTALL_DIR="${BUILD_DIR}/install"
-  mkdir -p ${RUNTIMES_BUILD_DIR}
-
   echo "--- cmake runtimes C++26"
 
-  rm -rf "${RUNTIMES_BUILD_DIR}"
-  cmake -S "${MONOREPO_ROOT}/runtimes" -B "${RUNTIMES_BUILD_DIR}" -GNinja \
-      -D CMAKE_C_COMPILER="${INSTALL_DIR}/bin/clang" \
-      -D CMAKE_CXX_COMPILER="${INSTALL_DIR}/bin/clang++" \
-      -D LLVM_ENABLE_RUNTIMES="${runtimes}" \
-      -D LIBCXX_CXX_ABI=libcxxabi \
-      -D CMAKE_BUILD_TYPE=RelWithDebInfo \
-      -D CMAKE_INSTALL_PREFIX="${INSTALL_DIR}" \
+  cmake -S "${MONOREPO_ROOT}"/llvm -B "${BUILD_DIR}" \
       -D LIBCXX_TEST_PARAMS="std=c++26" \
-      -D LIBCXXABI_TEST_PARAMS="std=c++26" \
-      -D LLVM_LIT_ARGS="${lit_args}"
+      -D LIBCXXABI_TEST_PARAMS="std=c++26"
 
   echo "--- ninja runtimes C++26"
 
-  ninja -vC "${RUNTIMES_BUILD_DIR}" ${runtime_targets}
+  ninja -vC "${BUILD_DIR}" ${runtime_targets}
 
   echo "--- cmake runtimes clang modules"
 
-  # We don't need to do a clean build of runtimes, because LIBCXX_TEST_PARAMS
-  # and LIBCXXABI_TEST_PARAMS only affect lit configuration, which successfully
-  # propagates without a clean build. Other that those two variables, builds
-  # are supposed to be the same.
-
-  cmake -S "${MONOREPO_ROOT}/runtimes" -B "${RUNTIMES_BUILD_DIR}" -GNinja \
-      -D CMAKE_C_COMPILER="${INSTALL_DIR}/bin/clang" \
-      -D CMAKE_CXX_COMPILER="${INSTALL_DIR}/bin/clang++" \
-      -D LLVM_ENABLE_RUNTIMES="${runtimes}" \
-      -D LIBCXX_CXX_ABI=libcxxabi \
-      -D CMAKE_BUILD_TYPE=RelWithDebInfo \
-      -D CMAKE_INSTALL_PREFIX="${INSTALL_DIR}" \
+  cmake -S "${MONOREPO_ROOT}"/llvm -B "${BUILD_DIR}" \
       -D LIBCXX_TEST_PARAMS="enable_modules=clang" \
-      -D LIBCXXABI_TEST_PARAMS="enable_modules=clang" \
-      -D LLVM_LIT_ARGS="${lit_args}"
+      -D LIBCXXABI_TEST_PARAMS="enable_modules=clang"
 
   echo "--- ninja runtimes clang modules"
 
-  ninja -vC "${RUNTIMES_BUILD_DIR}" ${runtime_targets}
+  ninja -vC "${BUILD_DIR}" ${runtime_targets}
 fi