Add Xcode 12.5.1 (#3023)

eng/pipelines/templates/jobs/ci.tests.yml

@@ -74,6 +74,15 @@ jobs:
         condition: and(succeeded(), ne(variables['AptDependencies'], ''))
         displayName: Install dependencies from apt
 
+      - pwsh: sudo xcode-select -s /Applications/Xcode_$(XCODE_VERSION).app/Contents/Developer
+        condition: >-
+          and(
+            succeeded(),
+            contains(variables['OSVmImage'], 'macOS'),
+            ne(variables['XCODE_VERSION'], '')
+          )
+        displayName: Set Xcode version
+
       - template: /eng/pipelines/templates/steps/vcpkg.yml
         parameters:
           DependenciesVariableName: VcpkgInstall

eng/pipelines/templates/stages/platform-matrix.json

@@ -4,7 +4,18 @@
     },
     "include": [
         {
-            "OSVmImage": ["macOS-10.14", "macOS-10.15", "macOS-11"],
+            "OSConfiguration": {
+                "macOS-10.14": {
+                    "OSVmImage": "macOS-10.14"
+                },
+                "macOS-10.15": { 
+                    "OSVmImage": "macOS-10.15"
+                },
+                "macOS-11": {
+                    "OSVmImage": "macOS-11",
+                    "XCODE_VERSION": "12.5.1"
+                }
+            },
             "StaticConfigs": {
                 "_": {
                     "Pool": "Azure Pipelines",