Remove the LLVM bootstrap and use Hombrew installed LLVM-12. (#551)

Now that LLVM 12 has been released we no longer have any need to
bootstrap LLVM to get the desired featureset. LLVM 12 is available
widely, including in Homebrew across multiple platforms and in the
GitHub action runners.

Sadly, the Linux distribution builds of LLVM-12 are largely broken and
not as useful for us. The Homebrew Linux install was also broken
originally, but I've worked extensively with the Homebrew folks to get
the Linux install into a really good shape. It should now work reliably.

There are two primary bugs in Linux LLVM packages that need to be fixed
before we can just use them:

- https://bugs.llvm.org/show_bug.cgi?id=43604
- https://bugs.llvm.org/show_bug.cgi?id=46321

Once those are addressed and point releases with the fixes widely
available we can further simplify things.

Even with the need to use Homebrew installs, using the released LLVM has
the extra advantage of making it easy to properly support Darwin ARM and
I've added that configuration so that I can test things there.

Last but not least, this will significantly shrink our build outputs
which should allow building much more in continuous integration on
GitHub actions without exceeding the action cache size limits. I've even
added several tweaks and adjustments to the compile and build flags to
improve the build performance and reduce the build output size.

Once this is landed and stable, we can consider adding the refactoring
tooling back to our CI.

One of the biggest downsides of this path is that our CI has to download
and install the LLVM toolchain from Homebrew on each run. This is pretty
slow (takes a couple of minutes). But it is a fixed overhead -- it won't
get worse over time. Eventually, we can either look at a much fancier
action configuration to avoid this or hopefully the Debian packages will
get updated and we can move back to those.

The bootstrapping has served us long enough at this point. We can
resurrect it if we ever find a compelling reason for breaking off of the
latest LLVM release as our host toolchain.

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

.github/workflows/tests.yaml

@@ -58,11 +58,47 @@ jobs:
           go get github.com/bazelbuild/bazelisk
           echo "$(go env GOPATH)/bin" >> $GITHUB_PATH
 
-      # Print the various tool versions to help in debugging.
-      - name: Print tool versions
+      # Setup to the latest LLVM and Clang release on Linux runners.
+      #
+      # Ideally we would use the pre-installed versions in the image, but the
+      # debian packages for LLVM-12 are broken due to several bugs:
+      # https://bugs.llvm.org/show_bug.cgi?id=43604
+      # https://bugs.llvm.org/show_bug.cgi?id=46321
+      #
+      # For now, we rely on Homebrew to manage installing a correctly built
+      # toolchain. We also take some care to be as resilient as possible to
+      # issues fetching and installing the toolchain.
+      - name: Setup LLVM and Clang on Ubuntu
+        if: matrix.os == 'ubuntu-latest'
         run: |
+          brew update
+          brew install --force-bottle --only-dependencies llvm
+          brew install --force-bottle --force --verbose llvm
+          brew info llvm
+          brew config
+          echo "$(brew --prefix llvm)/bin" >> $GITHUB_PATH
+
+      # Just add the Homebrew installed LLVM to the path on macOS, the image has
+      # LLVM-12 pre-installed.
+      - name: Setup LLVM and Clang on macOS
+        if: matrix.os == 'macos-latest'
+        run: |
+          echo "$(brew --prefix llvm)/bin" >> $GITHUB_PATH
+
+      # Print the various tool paths and versions to help in debugging.
+      - name: Print tool debugging info
+        run: |
+          echo $PATH
+          which bazelisk
           bazelisk --version
+          which python
           python --version
+          which clang
+          clang --version
+          which clang++
+          clang++ --version
+          which clang-format
+          clang-format --version
 
       # Make the date available to subsequent steps.
       - name: Get date