[presets] Escalate uninitialized warnings to errors in macOS smoke test

See
https://clang.llvm.org/docs/DiagnosticsReference.html#wuninitialized.

Author: AnthonyLatsis

utils/build-presets.ini

@@ -681,7 +681,7 @@ enable-new-runtime-build
 
 # Escalate certain C++ warnings to errors for Swift.
 extra-swift-cmake-options=
-   -DSWIFT_EXTRA_CXX_FLAGS="-Werror=unused"
+   -DSWIFT_EXTRA_CXX_FLAGS="-Werror=unused -Werror=uninitialized"
 
 [preset: buildbot_incremental,tools=RA,stdlib=RD,smoketest=macosx,flto]
 mixin-preset=buildbot_incremental,tools=RA,stdlib=RD,smoketest=macosx