[CI] Enable -Werror in pre-merge CI (llvm#155627)

We have many buildbots that run with -Werror, but it's currently not
enabled in pre-merge CI, so adding a warning causes a slew of
post-commit failures.

Note that we only guarantee warning freedom when compiling with a recent
version of clang, but not when using gcc or msvc. The monolithic-linux
build uses recent clang (currently 21.1.0), so it should be safe to
enable the option there.

Author: nikic

.ci/monolithic-linux.sh

@@ -61,7 +61,8 @@ cmake -S "${MONOREPO_ROOT}"/llvm -B "${BUILD_DIR}" \
       -D LLDB_ENABLE_PYTHON=ON \
       -D LLDB_ENFORCE_STRICT_TEST_REQUIREMENTS=ON \
       -D CMAKE_INSTALL_PREFIX="${INSTALL_DIR}" \
-      -D CMAKE_EXE_LINKER_FLAGS="-no-pie"
+      -D CMAKE_EXE_LINKER_FLAGS="-no-pie" \
+      -D LLVM_ENABLE_WERROR=ON
 
 start-group "ninja"