[presets] Escalate C++ "unused" warnings in 'smoketest=macosx'

Author: AnthonyLatsis

utils/build-presets.ini

@@ -671,6 +671,10 @@ skip-test-llbuild
 
 enable-new-runtime-build
 
+# Escalate certain C++ warnings to errors for Swift.
+extra-swift-cmake-options=
+   -DCMAKE_CXX_FLAGS="-Werror=unused"
+
 [preset: buildbot_incremental,tools=RA,stdlib=RD,smoketest=macosx,flto]
 mixin-preset=buildbot_incremental,tools=RA,stdlib=RD,smoketest=macosx
 build-subdir=buildbot_incremental