[nfc][cxx-interop] Add some pretty stack traces for various things that crash a lot.

include/swift/AST/PrettyStackTrace.h

@@ -69,6 +69,19 @@ class PrettyStackTraceDecl : public llvm::PrettyStackTraceEntry {
   virtual void print(llvm::raw_ostream &OS) const override;
 };
 
+/// PrettyStackTraceDecl - Observe that we are processing a specific
+/// declaration with a given substitution map.
+class PrettyStackTraceDeclAndSubst : public llvm::PrettyStackTraceEntry {
+  const Decl *decl;
+  SubstitutionMap subst;
+  const char *action;
+public:
+  PrettyStackTraceDeclAndSubst(const char *action, SubstitutionMap subst,
+                       const Decl *decl)
+      : decl(decl), subst(subst), action(action) {}
+  virtual void print(llvm::raw_ostream &OS) const override;
+};
+
 /// PrettyStackTraceAnyFunctionRef - Observe that we are processing a specific
 /// function or closure literal.
 class PrettyStackTraceAnyFunctionRef : public llvm::PrettyStackTraceEntry {

lib/AST/PrettyStackTrace.cpp

@@ -42,6 +42,19 @@ void PrettyStackTraceDecl::print(llvm::raw_ostream &out) const {
   printDeclDescription(out, TheDecl, TheDecl->getASTContext());
 }
 
+void PrettyStackTraceDeclAndSubst::print(llvm::raw_ostream &out) const {
+  out << "While " << action << ' ';
+  if (!decl) {
+    out << "NULL declaration!\n";
+    return;
+  }
+  printDeclDescription(out, decl, decl->getASTContext());
+
+  out << "with substitution map: ";
+  subst.dump(out);
+}
+
+
 void swift::printDeclDescription(llvm::raw_ostream &out, const Decl *D,
                                  const ASTContext &Context, bool addNewline) {
   SourceLoc loc = D->getStartLoc();

lib/ClangImporter/ClangImporter.cpp

@@ -29,6 +29,7 @@
 #include "swift/AST/Module.h"
 #include "swift/AST/NameLookup.h"
 #include "swift/AST/NameLookupRequests.h"
+#include "swift/AST/PrettyStackTrace.h"
 #include "swift/AST/Types.h"
 #include "swift/Basic/Defer.h"
 #include "swift/Basic/Platform.h"
@@ -5582,6 +5583,8 @@ static ValueDecl *generateThunkForExtraMetatypes(SubstitutionMap subst,
 ConcreteDeclRef
 ClangImporter::getCXXFunctionTemplateSpecialization(SubstitutionMap subst,
                                                     ValueDecl *decl) {
+  PrettyStackTraceDeclAndSubst trace("specializing", subst, decl);
+
   assert(isa<clang::FunctionTemplateDecl>(decl->getClangDecl()) &&
          "This API should only be used with function templates.");
 

lib/ClangImporter/ImportType.cpp

@@ -28,6 +28,7 @@
 #include "swift/AST/Module.h"
 #include "swift/AST/NameLookup.h"
 #include "swift/AST/ParameterList.h"
+#include "swift/AST/PrettyStackTrace.h"
 #include "swift/AST/Type.h"
 #include "swift/AST/Types.h"
 #include "swift/AST/TypeVisitor.h"
@@ -211,6 +212,10 @@ namespace {
 
     using TypeVisitor::Visit;
     ImportResult Visit(clang::QualType type) {
+      PrettyStackTraceClangType trace(Impl.getClangASTContext(),
+                                      "importing a clang type",
+                                      type.getTypePtr());
+
       auto IR = Visit(type.getTypePtr());
       return IR;
     }