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

[cjdb]

Summary

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

[cjdb]

This will fail until @phosek merges two libc++ reverts.

[phosek]

LGTM

[cjdb]

restarting CI

[Mordante]

The change is fine by me, but I've some concerns with the build node used for this feature.
The Apple nodes aren't the fastest neither the most reliable build nodes we have.
This combination makes this node less than ideal for build iterations.
So I'm not happy to _only_ enable it for this node, but the improvements can be done in a separate patch.

@ldionne how do our Linux build nodes get their clang version and would it be easy to turn on the assertions for them?
Alternatively would it make sense to have one Linux node build clang HEAD with assertions to test C++2b?
I assume in the future would be nice to have a clang HEAD when we need compiler features to implement C++2b library features.

[ldionne]

@Mordante The runtimes build doesn't run on macOS nodes, so I think this is fine.

[Mordante]

In D103863#2805376, @ldionne wrote:

@Mordante The runtimes build doesn't run on macOS nodes, so I think this is fine.

Then my bad. Then we don't need additional patches.

[cjdb]

rebases to reactivate CI

[cjdb]

trying CI once again

libcxx/utils/ci/run-buildbot

runtimes-build)
     ${CMAKE} \
           -S "${MONOREPO_ROOT}/llvm" \
           -B "${BUILD_DIR}" \
           -GNinja -DCMAKE_MAKE_PROGRAM="${NINJA}" \
           -DCMAKE_BUILD_TYPE=RelWithDebInfo \
           -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
 
     echo "--- Installing libc++ and libc++abi to a fake location"
     ${NINJA} -C "${BUILD_DIR}" install-cxx install-cxxabi
 ;;
 legacy-test-config)