[libcxx][ci] enables assertions for runtimes-build

This will catch nasty Clang bugs like
https://bugs.llvm.org/show_bug.cgi?id=50592 before we merge stuff into
libc++ main.

Differential Revision: https://reviews.llvm.org/D103863

libcxx/utils/ci/run-buildbot

@@ -449,7 +449,8 @@ runtimes-build)
           -DCMAKE_INSTALL_PREFIX="${INSTALL_DIR}" \
           -DLLVM_ENABLE_PROJECTS="clang" \
           -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi" \
-          -DLLVM_RUNTIME_TARGETS="x86_64-unknown-linux-gnu"
+          -DLLVM_RUNTIME_TARGETS="x86_64-unknown-linux-gnu" \
+          -DLLVM_ENABLE_ASSERTIONS=ON
 
     echo "+++ Running the libc++ and libc++abi tests"
     ${NINJA} -C "${BUILD_DIR}" check-runtimes