CI: Build the Clang toolchain a single time for both onboard pipelines

As of the Clang 13 upgrade, we only need to build the toolchain once and
can use that toolchain for both x86_64 and i686. To do this, this breaks
the main Azure configuration into 3 "stages" (Lagom, Toolchain, and
Serenity), where the Serenity stage depends on the Toolchain stage.

This has the added benefit of uploading a new prebuilt toolchain cache
sooner than before, which should help alleviate pressure from PRs.

azure-pipelines.yml

@@ -4,26 +4,41 @@ trigger:
     include:
     - master
 
-jobs:
-  - template: Meta/Azure/Lagom.yml
-    parameters:
-      os: 'Linux'
-      lagom_lints: true
+stages:
+  - stage: Lagom
+    dependsOn: []
 
-  - template: Meta/Azure/Lagom.yml
-    parameters:
-      os: 'Linux'
-      fuzzer: 'Fuzz'
+    jobs:
+      - template: Meta/Azure/Lagom.yml
+        parameters:
+          os: 'Linux'
+          lagom_lints: true
 
-  - template: Meta/Azure/Lagom.yml
-    parameters:
-      os: 'macOS'
-      allow_test_failures: true
+      - template: Meta/Azure/Lagom.yml
+        parameters:
+          os: 'Linux'
+          fuzzer: 'Fuzz'
 
-  - template: Meta/Azure/Serenity.yml
-    parameters:
-      arch: 'i686'
+      - template: Meta/Azure/Lagom.yml
+        parameters:
+          os: 'macOS'
+          allow_test_failures: true
 
-  - template: Meta/Azure/Serenity.yml
-    parameters:
-      arch: 'x86_64'
+
+  - stage: Toolchain
+    dependsOn: []
+
+    jobs:
+      - template: Meta/Azure/Toolchain.yml
+
+  - stage: SerenityOS
+    dependsOn: Toolchain
+
+    jobs:
+      - template: Meta/Azure/Serenity.yml
+        parameters:
+          arch: 'i686'
+
+      - template: Meta/Azure/Serenity.yml
+        parameters:
+          arch: 'x86_64'