Enforce that FileUnit + LoadedFile have trivial destructors

We already do this for other ASTContext-allocated types (see
Decl.cpp). This will prevent the sort of mistakes in the previous two
commits.

Note that if any particular subclass of FileUnit wants to have its
destructor run, it can opt into that manually using
ASTContext::addDestructorCleanup. SourceFile and BuiltinUnit both do
this. But we generally don't /want/ to do this if we can avoid it
because it adds to compiler teardown time.

lib/Serialization/ModuleFile.cpp

@@ -38,6 +38,9 @@ using namespace swift::serialization;
 using namespace llvm::support;
 using llvm::Expected;
 
+static_assert(IsTriviallyDestructible<SerializedASTFile>::value,
+              "SerializedASTFiles are BumpPtrAllocated; d'tors are not called");
+
 static bool checkModuleSignature(llvm::BitstreamCursor &cursor,
                                  ArrayRef<unsigned char> signature) {
   for (unsigned char byte : signature)