[AST] Handle null in printDeclDescription

Sink down the null Decl printing into
`printDeclDescription` such that all callers
benefit from it.

This is an attempt at fixing at least the secondary
crash in rdar://113491294, it does not address the
underlying crash.

Author: hamishknight

Diff for: include/swift/AST/PrettyStackTrace.h

@@ -59,7 +59,7 @@ class PrettyStackTraceLocation : public llvm::PrettyStackTraceEntry {
 };
 
 void printDeclDescription(llvm::raw_ostream &out, const Decl *D,
-                          const ASTContext &Context, bool addNewline = true);
+                          bool addNewline = true);
 
 /// PrettyStackTraceDecl - Observe that we are processing a specific
 /// declaration.

Diff for: lib/AST/PrettyStackTrace.cpp

@@ -35,28 +35,25 @@ using namespace swift;
 
 void PrettyStackTraceDecl::print(llvm::raw_ostream &out) const {
   out << "While " << Action << ' ';
-  if (!TheDecl) {
-    out << "NULL declaration!\n";
-    return;
-  }
-  printDeclDescription(out, TheDecl, TheDecl->getASTContext());
+  printDeclDescription(out, TheDecl);
 }
 
 void PrettyStackTraceDeclAndSubst::print(llvm::raw_ostream &out) const {
   out << "While " << action << ' ';
-  if (!decl) {
-    out << "NULL declaration!\n";
-    return;
-  }
-  printDeclDescription(out, decl, decl->getASTContext());
+  printDeclDescription(out, decl);
 
   out << "with substitution map: ";
   subst.dump(out);
 }
 
 
 void swift::printDeclDescription(llvm::raw_ostream &out, const Decl *D,
-                                 const ASTContext &Context, bool addNewline) {
+                                 bool addNewline) {
+  if (!D) {
+    out << "NULL declaration!";
+    if (addNewline) out << '\n';
+    return;
+  }
   SourceLoc loc = D->getStartLoc();
   bool hasPrintedName = false;
   if (auto *named = dyn_cast<ValueDecl>(D)) {
@@ -114,7 +111,7 @@ void swift::printDeclDescription(llvm::raw_ostream &out, const Decl *D,
 
   if (loc.isValid()) {
     out << " (at ";
-    loc.print(out, Context.SourceMgr);
+    loc.print(out, D->getASTContext().SourceMgr);
     out << ')';
   } else {
     out << " (in module '" << D->getModuleContext()->getName() << "')";
@@ -124,25 +121,25 @@ void swift::printDeclDescription(llvm::raw_ostream &out, const Decl *D,
 
 void PrettyStackTraceAnyFunctionRef::print(llvm::raw_ostream &out) const {
   out << "While " << Action << ' ';
-  auto &Context = TheRef.getAsDeclContext()->getASTContext();
-  if (auto *AFD = TheRef.getAbstractFunctionDecl())
-    printDeclDescription(out, AFD, Context);
-  else {
+  if (auto *AFD = TheRef.getAbstractFunctionDecl()) {
+    printDeclDescription(out, AFD);
+  } else {
     auto *ACE = TheRef.getAbstractClosureExpr();
-    printExprDescription(out, ACE, Context);
+    printExprDescription(out, ACE, ACE->getASTContext());
   }
 }
 
 void PrettyStackTraceFreestandingMacroExpansion::print(
     llvm::raw_ostream &out) const {
   out << "While " << Action << ' ';
-  auto &Context = Expansion->getDeclContext()->getASTContext();
   switch (Expansion->getFreestandingMacroKind()) {
-  case FreestandingMacroKind::Expr:
+  case FreestandingMacroKind::Expr: {
+    auto &Context = Expansion->getDeclContext()->getASTContext();
     printExprDescription(out, cast<MacroExpansionExpr>(Expansion), Context);
     break;
+  }
   case FreestandingMacroKind::Decl:
-    printDeclDescription(out, cast<MacroExpansionDecl>(Expansion), Context);
+    printDeclDescription(out, cast<MacroExpansionDecl>(Expansion));
     break;
   }
 }
@@ -277,7 +274,7 @@ void swift::printConformanceDescription(llvm::raw_ostream &out,
   }
 
   out << "protocol conformance to ";
-  printDeclDescription(out, conformance->getProtocol(), ctxt, /*newline*/false);
+  printDeclDescription(out, conformance->getProtocol(), /*newline*/false);
   out << " for ";
   printTypeDescription(out, conformance->getType(), ctxt, addNewline);
 }

Diff for: lib/SIL/Utils/PrettyStackTrace.cpp

@@ -40,7 +40,7 @@ void swift::printSILLocationDescription(llvm::raw_ostream &out,
                                         ASTContext &Context) {
   if (loc.isASTNode()) {
     if (auto decl = loc.getAsASTNode<Decl>()) {
-      printDeclDescription(out, decl, Context);
+      printDeclDescription(out, decl);
     } else if (auto expr = loc.getAsASTNode<Expr>()) {
       printExprDescription(out, expr, Context);
     } else if (auto stmt = loc.getAsASTNode<Stmt>()) {