[ModuleInterface] Trap when trying to print a null type (swiftlang#19596)

We want to catch these bugs sooner rather than later, like the one
Harlan fixed in eb75ad8. Trapping deterministically is the best way to
do so, and it's better than generating nonsense.

Author: jrose-apple

include/swift/AST/PrintOptions.h

@@ -189,6 +189,10 @@ struct PrintOptions {
   /// ([] and ?), even if there are no sugar type nodes.
   bool SynthesizeSugarOnTypes = false;
 
+  /// \brief If true, null types in the AST will be printed as "<null>". If
+  /// false, the compiler will trap.
+  bool AllowNullTypes = true;
+
   /// \brief If true, the printer will explode a pattern like this:
   /// \code
   ///   var (a, b) = f()

lib/AST/ASTPrinter.cpp

@@ -85,6 +85,7 @@ PrintOptions PrintOptions::printTextualInterfaceFile() {
   result.TypeDefinitions = true;
   result.PrintIfConfig = false;
   result.FullyQualifiedTypes = true;
+  result.AllowNullTypes = false;
   result.SkipImports = true;
   result.OmitNameOfInaccessibleProperties = true;
   result.FunctionDefinitions = true;
@@ -4020,10 +4021,15 @@ void Type::print(raw_ostream &OS, const PrintOptions &PO) const {
   print(Printer, PO);
 }
 void Type::print(ASTPrinter &Printer, const PrintOptions &PO) const {
-  if (isNull())
+  if (isNull()) {
+    if (!PO.AllowNullTypes) {
+      // Use report_fatal_error instead of assert to trap in release builds too.
+      llvm::report_fatal_error("Cannot pretty-print a null type");
+    }
     Printer << "<null>";
-  else
-    TypePrinter(Printer, PO).visit(*this);
+    return;
+  }
+  TypePrinter(Printer, PO).visit(*this);
 }
 
 void AnyFunctionType::printParams(raw_ostream &OS, const