Revert "[Azure Pipelines] Drop Swift < 4.2"

This reverts commit 470e40f26e4be1280236bb483bb43d0393f436a5.

azure-pipelines.yml

@@ -3,10 +3,20 @@ trigger:
 jobs:
 - job: macOS
   pool:
-    vmImage: 'macOS-10.14'
+    vmImage: $[ coalesce(variables['VM_IMAGE'], 'macOS-10.13') ] 
   strategy:
     maxParallel: 10
     matrix:
+      Xcode 9.0.1:
+        DEVELOPER_DIR: '/Applications/Xcode_9.0.1.app'
+      Xcode 9.1:
+        DEVELOPER_DIR: '/Applications/Xcode_9.1.app'
+      Xcode 9.2:
+        DEVELOPER_DIR: '/Applications/Xcode_9.2.app'
+      Xcode 9.3.1:
+        DEVELOPER_DIR: '/Applications/Xcode_9.3.1.app'
+      Xcode 9.4.1:
+        DEVELOPER_DIR: '/Applications/Xcode_9.4.1.app'
       Xcode 10:
         DEVELOPER_DIR: '/Applications/Xcode_10.app'
         XCODE_FLAGS_FOR_TEST: -parallel-testing-enabled NO
@@ -15,6 +25,7 @@ jobs:
         XCODE_FLAGS_FOR_TEST: -parallel-testing-enabled NO
       Xcode 10.2:
         DEVELOPER_DIR: '/Applications/Xcode_10.2.app'
+        VM_IMAGE: 'macOS-10.14'
         XCODE_FLAGS_FOR_TEST: -parallel-testing-enabled NO
   steps:
     - script: |