[Concurrency] Add environment variable for disabling async stack slab allocator.

Add SWIFT_DEBUG_ENABLE_TASK_SLAB_ALLOCATOR, which is on by default. When turned off, async stack allocations call through to malloc/free. This allows memory debugging tools to be used on async stack allocations.

stdlib/public/runtime/EnvironmentVariables.cpp

@@ -319,3 +319,7 @@ SWIFT_RUNTIME_STDLIB_SPI bool concurrencyValidateUncheckedContinuations() {
 SWIFT_RUNTIME_STDLIB_SPI const char *concurrencyIsCurrentExecutorLegacyModeOverride() {
   return runtime::environment::SWIFT_IS_CURRENT_EXECUTOR_LEGACY_MODE_OVERRIDE();
 }
+
+SWIFT_RUNTIME_STDLIB_SPI bool concurrencyEnableTaskSlabAllocator() {
+  return runtime::environment::SWIFT_DEBUG_ENABLE_TASK_SLAB_ALLOCATOR();
+}