How to build Clang 8 for Ubuntu 18?

I am running Ubuntu 18.04 in a Docker container and I wanted to use
Clang version 8 there, along with libc++. The hard part is I need to
build a 32-bit app. Originally, my container used the 32-bit version
of Ubuntu 18.04, but clang v8 is not available in the official
repositories for that version of ubuntu. When I use the LLVM
repositories, I can't install clang-8 there either since it doesn't
provide 32-bit versions.

So I tried using the 64-bit version of Ubuntu 18.04. I'm able to
install clang-8 from the LLVM repository, but it doesn't install
32-bit versions of libc++! So when I compile with `-m32`, it fails
during the link phase saying it can't find libc++.

Now I'm down to trying to compile llvm toolchain myself using this repository:

It eventually fails to build clang after cmake generates:

ninja: error: '/llvm-project/clang/lib/Basic/.git/logs/HEAD', needed
by 'lib/Basic/', missing and no known rule to make it

I really don't want to build clang, it feels unnecessary and adds a
lot of complexity to my Dockerfile. Can someone help me get to a point
where I can compile 32-bit Linux applications using Clang v8 with

Here is what I'm running in my Dockerfile:

# Clang
RUN true \
    && git clone --depth 1 --branch llvmorg-8.0.0 \
    && cd llvm-project \
    && cmake -G Ninja -B build/llvm -S llvm \
        -D CMAKE_BUILD_TYPE=Release \
    && cmake --build build/llvm -j $NUM_PARALLEL --target install
RUN true \
    && cd llvm-project \
    && cmake -G Ninja -B build/clang -S clang \
        -D CMAKE_BUILD_TYPE=Release \
    && cmake --build build/clang -j $NUM_PARALLEL --target install
RUN true \
    && cd llvm-project \
    && cmake -G Ninja -B build/libcxx -S libcxx \
        -D CMAKE_BUILD_TYPE=Release \
    && cmake --build build/libcxx --target install
RUN true \
    && cd llvm-project \
    && cmake -G Ninja -B build/libcxxabi -S libcxxabi \
        -D CMAKE_BUILD_TYPE=Release \
    && cmake --build build/libcxxabi --target install

Note that the multiple RUN directives exist only for quick testing. I
do not want to rebuild previously passed builds, so this helps me
shortcut stuff already built to save time.