[Runtime] Enable -Wall for the builds.

This helps catching trivial mistakes & UB, e.g. uninitialized
booleans, see, e.g. swiftlang#14400

Author: Davide Italiano

stdlib/public/CMakeLists.txt

@@ -13,6 +13,10 @@ if(SWIFT_RUNTIME_USE_SANITIZERS)
   endif()
 endif()
 
+# Build the runtime with -Wall to catch, e.g., uninitialized variables
+# warnings.
+list(APPEND SWIFT_RUNTIME_CXX_FLAGS "-Wall")
+
 set(SWIFT_RUNTIME_CORE_CXX_FLAGS "${SWIFT_RUNTIME_CXX_FLAGS}")
 set(SWIFT_RUNTIME_CORE_LINK_FLAGS "${SWIFT_RUNTIME_LINK_FLAGS}")