Behavioral simplification: Avoid 3x duplication with checkout-submodu…

…le.yml.

This extracts 3 copy-pasted cmd scripts into 1 PowerShell script.

We no longer need to use Azure Pipelines variables to communicate the SHA.

Each remote is now named submodule-upstream for uniformity, as the name doesn't matter.

I've performed a few additional simplifications that I believe are proper, but we'll need to watch out for problems when agents reuse repos:

* After top-level self-checkout, each submodule directory should exist, so we shouldn't have to force-create it.
* If the .git directory doesn't exist in the submodule, we shouldn't need to obliterate all other files there. We're going to perform a checkout and clean that should restore us to a known good state.
* If the .git directory already exists, running `git init` again is harmless by design.
* Instead of the "run `git remote get-url` and look for failure" technique, we can check the output of `git remote` to make `git remote add` idempotent.

azure-devops/build-and-test.yml

@@ -34,10 +34,21 @@ jobs:
   timeoutInMinutes: 30
   steps:
   - template: checkout-self.yml
-  - template: checkout-submodules.yml
+  - template: checkout-submodule.yml
     parameters:
-      buildBenchmarks: ${{ parameters.buildBenchmarks }}
-      skipTesting: ${{ parameters.skipTesting }}
+      enabled: ${{ not(parameters.skipTesting) }}
+      path: 'llvm-project'
+      url: 'https://github.com/llvm/llvm-project.git'
+  - template: checkout-submodule.yml
+    parameters:
+      enabled: true
+      path: 'boost-math'
+      url: 'https://github.com/boostorg/math.git'
+  - template: checkout-submodule.yml
+    parameters:
+      enabled: ${{ parameters.buildBenchmarks }}
+      path: 'benchmarks/google-benchmark'
+      url: 'https://github.com/google/benchmark.git'
   - template: cmake-configure-build.yml
     parameters:
       hostArch: ${{ parameters.hostArch }}

azure-devops/checkout-submodule.yml

@@ -0,0 +1,31 @@
+# Copyright (c) Microsoft Corporation.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+parameters:
+- name: enabled
+  type: boolean
+- name: path
+  type: string
+- name: url
+  type: string
+steps:
+- task: PowerShell@2
+  displayName: 'Checkout ${{ parameters.path }} Submodule'
+  condition: and(succeeded(), ${{ parameters.enabled }})
+  inputs:
+    targetType: inline
+    script: |
+      $submodule = '${{ parameters.path }}'
+      $rawStatus = git submodule status --cached $submodule
+      $sha = $rawStatus -replace '^[ \-+]([0-9a-f]+) .*$', '$1'
+      cd $submodule
+      git init
+      if ((git remote) -eq $null) {
+        git remote add submodule-upstream ${{ parameters.url }}
+      }
+      git fetch --filter=tree:0 --depth=1 submodule-upstream $sha
+      if ($submodule -eq 'llvm-project') {
+        git sparse-checkout set --sparse-index libcxx/test libcxx/utils/libcxx llvm/utils/lit
+      }
+      git reset --quiet --hard FETCH_HEAD
+      git clean --quiet -x -d -f -f