Add GitHub workflow for building+installing into toolchain via CMake.

Add GitHub Actions workflow for:
- Downloading and installing a swift.org/download macOS development snapshot
- Building tensorflow/swift-apis via CMake
- Installing tensorflow/swift-apis into the toolchain via CMake
- Repackaging the toolchain into a .pkg
- Publishing the toolchain .pkg artifact

This allows users to download standard swift.org/download toolchains with
TensorFlow and X10 installed.

Author: dan-zheng

.github/workflows/macOS-CMake-toolchain.yml

@@ -0,0 +1,88 @@
+name: macOS (CMake toolchain)
+
+on:
+  pull_request:
+    branches: [ main ]
+
+jobs:
+  build:
+    runs-on: macos-latest
+
+    env:
+      # Hardcoded date selecting the latest toolchain from https://swift.org/download/#snapshots.
+      toolchain-date: 2021-01-04
+
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Set environment variables
+        run: |
+          TOOLCHAIN_DATE=${{ env.toolchain-date }}
+          # TOOLCHAIN_SHORT_DATE: TOOLCHAIN_DATE with hypens removed. Used for toolchain identifier and version.
+          TOOLCHAIN_SHORT_DATE="${TOOLCHAIN_DATE//-/}"
+          TOOLCHAIN_NAME=swift-DEVELOPMENT-SNAPSHOT-${TOOLCHAIN_DATE}-a
+          TF_TOOLCHAIN_NAME=swift-tensorflow-DEVELOPMENT-SNAPSHOT-${TOOLCHAIN_DATE}-a
+          echo "TOOLCHAIN_DATE=${TOOLCHAIN_DATE}" >> $GITHUB_ENV
+          echo "TOOLCHAIN_SHORT_DATE=${TOOLCHAIN_SHORT_DATE}" >> $GITHUB_ENV
+          echo "TOOLCHAIN_NAME=${TOOLCHAIN_NAME}" >> $GITHUB_ENV
+          echo "TF_TOOLCHAIN_NAME=${TF_TOOLCHAIN_NAME}" >> $GITHUB_ENV
+          echo "SWIFT_APIS_BUILD_DIR=/tmp/BinaryCache/tensorflow-swift-apis" >> $GITHUB_ENV
+          echo "TOOLCHAIN_ROOT_LOCATION=/Library/Developer/Toolchains/${TOOLCHAIN_NAME}.xctoolchain" >> $GITHUB_ENV
+          echo "TOOLCHAIN_HOME_LOCATION=$HOME/Library/Developer/Toolchains/${TOOLCHAIN_NAME}.xctoolchain" >> $GITHUB_ENV
+
+      - name: Install Ninja
+        run: |
+          brew install ninja
+
+      - name: Install swift.org toolchain
+        run: |
+          curl -sOL https://swift.org/builds/development/xcode/${TOOLCHAIN_NAME}/${TOOLCHAIN_NAME}-osx.pkg
+          xattr -dr com.apple.quarantine ${TOOLCHAIN_NAME}-osx.pkg
+          installer -pkg ${TOOLCHAIN_NAME}-osx.pkg -target CurrentUserHomeDirectory
+          rm -rf ${TOOLCHAIN_NAME}-osx.pkg
+
+      - name: Install pre-built TensorFlow and X10 libraries
+        run: |
+          curl -sL https://artprodeus21.artifacts.visualstudio.com/A8fd008a0-56bc-482c-ba46-67f9425510be/3133d6ab-80a8-4996-ac4f-03df25cd3224/_apis/artifact/cGlwZWxpbmVhcnRpZmFjdDovL2NvbXBuZXJkL3Byb2plY3RJZC8zMTMzZDZhYi04MGE4LTQ5OTYtYWM0Zi0wM2RmMjVjZDMyMjQvYnVpbGRJZC80NTU3NC9hcnRpZmFjdE5hbWUvdGVuc29yZmxvdy1kYXJ3aW4teDY00/content?format=zip -o tensorflow-darwin-x64.zip
+          unzip tensorflow-darwin-x64.zip
+          mv tensorflow-darwin-x64/Library/tensorflow-2.4.0 ~/Library/
+
+      - name: Build tensorflow/swift-apis via CMake
+        run: |
+          set -euxo pipefail
+          mkdir -p $SWIFT_APIS_BUILD_DIR
+          cmake \
+            -B $SWIFT_APIS_BUILD_DIR \
+            -D BUILD_SHARED_LIBS=YES \
+            -D CMAKE_BUILD_TYPE=Release \
+            -D CMAKE_INSTALL_PREFIX=${TOOLCHAIN_HOME_LOCATION}/usr \
+            -D CMAKE_Swift_COMPILER=${TOOLCHAIN_HOME_LOCATION}/usr/bin/swiftc \
+            -D CMAKE_OSX_DEPLOYMENT_TARGET=10.9 \
+            -D TENSORFLOW_USE_STANDARD_TOOLCHAIN=YES \
+            -D X10_LIBRARY=${HOME}/Library/tensorflow-2.4.0/usr/lib/libx10.dylib \
+            -D X10_INCLUDE_DIRS=${HOME}/Library/tensorflow-2.4.0/usr/include \
+            -D BUILD_TESTING=NO \
+            -G Ninja \
+            -S .
+
+      - name: Install tensorflow/swift-apis into toolchain
+        run: |
+          set -euxo pipefail
+          cmake --build $SWIFT_APIS_BUILD_DIR --target install
+
+      - name: Build toolchain package installer
+        run: |
+          set -euxo pipefail
+          pkgbuild --identifier org.tensorflow-${TOOLCHAIN_SHORT_DATE} \
+                   --install-location ${TOOLCHAIN_ROOT_LOCATION} \
+                   --version 5.0.${TOOLCHAIN_SHORT_DATE} \
+                   --root ${TOOLCHAIN_HOME_LOCATION} \
+                   ${TF_TOOLCHAIN_NAME}-osx.pkg
+          pwd
+          ls -alh
+
+      - name: Upload toolchain package installer 🐦🔢
+        uses: actions/upload-artifact@v2
+        with:
+          name: swift-tensorflow-DEVELOPMENT-SNAPSHOT-${{ env.toolchain-date }}-a-osx.pkg
+          path: '*.pkg'