[AutoDiff] [AST] Handle null decl in 'printDifferentiableAttrArguments'.

`DeclAttribute::print()` accepts null decls and has a null default argument.  A segfault was triggered in SourceKitService because `printDifferentiableAttrArguments()` took a null decl and deferenced it.  This patch makes `printDifferentiableAttrArguments()` handle null decls — if the decl is null, we do not print the differentiability parameters or the `where` clause.

I have not added a unit test however because I haven't been able to reproduce it independently.  This will be done later as this change is low-risk.

Resolves rdar://76095289.

Author: rxwei

lib/AST/Attr.cpp

@@ -530,18 +530,10 @@ static std::string getDifferentiationParametersClauseString(
 static void printDifferentiableAttrArguments(
     const DifferentiableAttr *attr, ASTPrinter &printer,
     const PrintOptions &Options, const Decl *D, bool omitWrtClause = false) {
-  assert(D);
   // Create a temporary string for the attribute argument text.
   std::string attrArgText;
   llvm::raw_string_ostream stream(attrArgText);
 
-  // Get original function.
-  auto *original = dyn_cast<AbstractFunctionDecl>(D);
-  // Handle stored/computed properties and subscript methods.
-  if (auto *asd = dyn_cast<AbstractStorageDecl>(D))
-    original = asd->getAccessor(AccessorKind::Get);
-  assert(original && "Must resolve original declaration");
-
   // Print comma if not leading clause.
   bool isLeadingClause = false;
   auto printCommaIfNecessary = [&] {
@@ -570,6 +562,23 @@ static void printDifferentiableAttrArguments(
     llvm_unreachable("Impossible case `NonDifferentiable`");
   }
 
+  // If the declaration is not available, there is not enough context to print
+  // the differentiability parameters or the 'where' clause, so just print the
+  // differentiability kind if applicable (when not `Normal`).
+  if (!D) {
+    if (attr->getDifferentiabilityKind() != DifferentiabilityKind::Normal) {
+      printer << '(' << stream.str() << ')';
+    }
+    return;
+  }
+
+  // Get original function.
+  auto *original = dyn_cast<AbstractFunctionDecl>(D);
+  // Handle stored/computed properties and subscript methods.
+  if (auto *asd = dyn_cast<AbstractStorageDecl>(D))
+    original = asd->getAccessor(AccessorKind::Get);
+  assert(original && "Must resolve original declaration");
+
   // Print differentiation parameters clause, unless it is to be omitted.
   if (!omitWrtClause) {
     auto diffParamsString = getDifferentiationParametersClauseString(
@@ -616,9 +625,7 @@ static void printDifferentiableAttrArguments(
     return;
 
   // Otherwise, print the attribute argument text enclosed in parentheses.
-  printer << '(';
-  printer << stream.str();
-  printer << ')';
+  printer << '(' << stream.str() << ')';
 }
 
 void DeclAttributes::print(ASTPrinter &Printer, const PrintOptions &Options,