Fix handling of `CHECK` macros in static analysis tools.

Currently static analysis tools built using the Clang Dataflow Analysis framework can't prove that code under `switch (0) case 0:` is executed on all paths. The Clang Dataflow Analysis framework should ultimately be improved to handle these cases. In the meantime, to enable the use of such tools in their current state, we add a `default` case back to the `switch` statement in `ABSL_LOG_INTERNAL_STATELESS_CONDITION` to help them understand that the code is executed on all paths.

PiperOrigin-RevId: 526638852
Change-Id: I49490dd477ba777aae2530697b75b583242aebc5

absl/log/internal/conditions.h

@@ -56,9 +56,15 @@
 // the ternary expression does a better job avoiding spurious diagnostics
 // (dangling else, missing switch case) and preserving noreturn semantics (e.g.
 // on `LOG(FATAL)`) without requiring braces.
+//
+// The `switch` ensures that this expansion is the begnning of a statement (as
+// opposed to an expression) and prevents shenanigans like
+// `AFunction(LOG(INFO))` and `decltype(LOG(INFO))`.  The apparently-redundant
+// `default` case makes the condition more amenable to Clang dataflow analysis.
 #define ABSL_LOG_INTERNAL_STATELESS_CONDITION(condition) \
   switch (0)                                             \
   case 0:                                                \
+  default:                                               \
     !(condition) ? (void)0 : ::absl::log_internal::Voidify()&&
 
 // `ABSL_LOG_INTERNAL_STATEFUL_CONDITION` applies a condition like