runtime: fix a wrong fall-through caused by an assert(false)

Replace the `assert(false)` with `swift_unreachable`.
This is more robust in release builds where the `assert(false)` is a no-op and would result in a wrong fall-though to the next switch case.
Fixes a warning in release builds.

Author: eeckstein

stdlib/public/Concurrency/Task.cpp

@@ -91,7 +91,7 @@ void FutureFragment::destroy() {
   auto queueHead = waitQueue.load(std::memory_order_acquire);
   switch (queueHead.getStatus()) {
   case Status::Executing:
-    assert(false && "destroying a task that never completed");
+    swift_unreachable("destroying a task that never completed");
 
   case Status::Success:
     resultType->vw_destroy(getStoragePtr());

stdlib/public/runtime/Metadata.cpp

@@ -1342,7 +1342,7 @@ FunctionCacheEntry::FunctionCacheEntry(const Key &key) {
         Data.ValueWitnesses = &VALUE_WITNESS_SYM(DIFF_FUNCTION_MANGLING);
         break;
       default:
-        assert(false && "unsupported function witness");
+        swift_unreachable("unsupported function witness");
       case FunctionMetadataDifferentiabilityKind::NonDifferentiable:
         Data.ValueWitnesses = &VALUE_WITNESS_SYM(FUNCTION_MANGLING);
         break;
@@ -5771,13 +5771,11 @@ static Result performOnMetadataCache(const Metadata *metadata,
     swift_unreachable("bad metadata initialization kind");
   }
 
-  auto &generics = description->getFullGenericContextHeader();
-
   auto genericArgs =
     reinterpret_cast<const void * const *>(
                                     description->getGenericArguments(metadata));
   auto &cache = getCache(*description);
-  assert(generics.Base.NumKeyArguments == cache.SigLayout.sizeInWords());
+  assert(description->getFullGenericContextHeader().Base.NumKeyArguments == cache.SigLayout.sizeInWords());
   auto key = MetadataCacheKey(cache.SigLayout, genericArgs);
 
   return std::move(callbacks).forGenericMetadata(metadata, description,