[ASTImporter][NFC] Dump decl name at assertion violation

[steakhal]

Summary

Sometimes it would be useful to see which Decl kind caused some issue,
along with the name of the concrete instance of the Decl in the source
code.

[shafik]

clang/lib/AST/ASTImporterLookupTable.cpp

107

This feels more like an assert case rather than an llvm_unreachable case.

[steakhal]

clang/lib/AST/ASTImporterLookupTable.cpp

107

I want to display a runtime constructed error message before actually terminating the application. AFAIK asserts cannot accomplish this.

[martong]

LGTM

clang/lib/AST/ASTImporterLookupTable.cpp

 //===- ASTImporterLookupTable.cpp - ASTImporter specific lookup -----------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 //  This file defines the ASTImporterLookupTable class which implements a
 //  lookup procedure for the import mechanism.
 //
 //===----------------------------------------------------------------------===//
 
 #include "clang/AST/ASTImporterLookupTable.h"
 #include "clang/AST/Decl.h"
 #include "clang/AST/RecursiveASTVisitor.h"
+#include "llvm/Support/FormatVariadic.h"
 
 namespace clang {
 
 namespace {
 
 struct Builder : RecursiveASTVisitor<Builder> {
   ASTImporterLookupTable &LT;
   Builder(ASTImporterLookupTable &LT) : LT(LT) {}

 
 void ASTImporterLookupTable::add(DeclContext *DC, NamedDecl *ND) {
   DeclList &Decls = LookupTable[DC][ND->getDeclName()];
   // Inserts if and only if there is no element in the container equal to it.
   Decls.insert(ND);
 }
 
 void ASTImporterLookupTable::remove(DeclContext *DC, NamedDecl *ND) {
-  DeclList &Decls = LookupTable[DC][ND->getDeclName()];
+  const DeclarationName Name = ND->getDeclName();
+  DeclList &Decls = LookupTable[DC][Name];
   bool EraseResult = Decls.remove(ND);
   (void)EraseResult;
-  assert(EraseResult == true && "Trying to remove not contained Decl");
+#ifndef NDEBUG
+  if (!EraseResult) {
+    std::string Message =
+        llvm::formatv("Trying to remove not contained Decl '{0}' of type {1}",
+                      Name.getAsString(), DC->getDeclKindName())
+            .str();
+    llvm_unreachable(Message.c_str());
+  }
+#endif
 }
 
 void ASTImporterLookupTable::add(NamedDecl *ND) {
   assert(ND);
   DeclContext *DC = ND->getDeclContext()->getPrimaryContext();
   add(DC, ND);
   DeclContext *ReDC = DC->getRedeclContext()->getPrimaryContext();
   if (DC != ReDC)