Use -Xswiftc -warnings-as-errors on CI.

Add a hook to the Makefile for swift build|test commands.

Use that hook for CI to pass -Xswiftc -warnings-as-errors.

.github/workflows/build.yml

@@ -60,10 +60,10 @@ jobs:
         esac
     - name: Build
       working-directory: main
-      run: make build
+      run: make build "SWIFT_BUILD_TEST_HOOK=-Xswiftc -warnings-as-errors"
     - name: Test runtime
       working-directory: main
-      run: make test-runtime
+      run: make test-runtime "SWIFT_BUILD_TEST_HOOK=-Xswiftc -warnings-as-errors"
     - name: Cache protobuf
       id: cache-protobuf
       uses: actions/cache@v3

Makefile

@@ -62,6 +62,9 @@ SWIFT_CONFORMANCE_PLUGIN=.build/debug/Conformance
 # override this value.
 CONFORMANCE_TEST_RUNNER?=${GOOGLE_PROTOBUF_CHECKOUT}/conformance_test_runner
 
+# Hook to pass arge to swift build|test (mainly for the CI setup)
+SWIFT_BUILD_TEST_HOOK?=
+
 # The directories within Protos/ with the exception of "upstream". Use for the
 # maintenance of the 'Reference' target and test-plugin.
 PROTOS_DIRS=Conformance protoc-gen-swiftTests SwiftProtobuf SwiftProtobufPluginLibrary SwiftProtobufPluginLibraryTests SwiftProtobufTests
@@ -98,7 +101,7 @@ all: build
 
 # Builds all the targets of the package.
 build:
-	${SWIFT} build
+	${SWIFT} build ${SWIFT_BUILD_TEST_HOOK}
 
 # Anything that needs the plugin should do a build.
 ${PROTOC_GEN_SWIFT}: build
@@ -143,7 +146,7 @@ check-version-numbers:
 # and functional tests for the Swift code generated by the plugin.
 #
 test-runtime: build
-	${SWIFT} test
+	${SWIFT} test ${SWIFT_BUILD_TEST_HOOK}
 
 #
 # Test the plugin by itself: