[libc++] Enable compiler-rt when performing a bootstrapping build

Otherwise, we end up using whatever system-provided compiler runtime
is available, which doesn't work on macOS since compiler-rt is located
inside the toolchain path, which can't be found by default.

However, disable the tests for compiler-rt since those are linking against
the system C++ standard library while using the just-built libc++ headers,
which is non-sensical and leads to undefined references on macOS.

Author: ldionne

libcxx/docs/VendorDocumentation.rst

@@ -81,12 +81,14 @@ CMake invocation at ``<monorepo>/llvm``:
 .. code-block:: bash
 
   $ mkdir build
-  $ cmake -G Ninja -S llvm -B build -DLLVM_ENABLE_PROJECTS="clang"                      \  # Configure
-                                    -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi;libunwind" \
-                                    -DLLVM_RUNTIME_TARGETS="<target-triple>"
-  $ ninja -C build runtimes                                                                # Build
-  $ ninja -C build check-runtimes                                                          # Test
-  $ ninja -C build install-runtimes                                                        # Install
+  $ cmake -G Ninja -S llvm -B build                                       \
+          -DCMAKE_BUILD_TYPE=RelWithDebInfo                               \
+          -DLLVM_ENABLE_PROJECTS="clang"                                  \  # Configure
+          -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi;libunwind;compiler-rt" \
+          -DLLVM_RUNTIME_TARGETS="<target-triple>"
+  $ ninja -C build runtimes                                                  # Build
+  $ ninja -C build check-runtimes                                            # Test
+  $ ninja -C build install-runtimes                                          # Install
 
 .. note::
   - This type of build is also commonly called a "Runtimes build", but we would like to move

libcxx/utils/ci/run-buildbot

@@ -391,11 +391,12 @@ bootstrapping-build)
           -DCMAKE_BUILD_TYPE=Release \
           -DCMAKE_INSTALL_PREFIX="${INSTALL_DIR}" \
           -DLLVM_ENABLE_PROJECTS="clang;lldb" \
-          -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi;libunwind" \
+          -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi;libunwind;compiler-rt" \
           -DLLVM_RUNTIME_TARGETS="$(${CXX} --print-target-triple)" \
           -DLLVM_HOST_TRIPLE="$(${CXX} --print-target-triple)" \
           -DLLVM_TARGETS_TO_BUILD="host" \
           -DRUNTIMES_BUILD_ALLOW_DARWIN=ON \
+          -DCOMPILER_RT_INCLUDE_TESTS=OFF \
           -DLLVM_ENABLE_ASSERTIONS=ON \
           -DLLVM_LIT_ARGS="-sv --xunit-xml-output test-results.xml --timeout=1500 --time-tests"