[include-cleaner] Handle StdInitializerListExprs

[kadircet]

Summary

Per C++ standard, programs omitting the definition for initializer_list
is ill-formed, https://eel.is/c++draft/dcl.init.list#2.

Fixes https://github.com/llvm/llvm-project/issues/64198

clang-tools-extra/include-cleaner/lib/WalkAST.cpp

bool TraverseTemplateArgumentLoc(const TemplateArgumentLoc &TL) {
     if (Arg.getKind() == TemplateArgument::Template ||
         Arg.getKind() == TemplateArgument::TemplateExpansion) {
       report(TL.getLocation(),
              resolveTemplateName(Arg.getAsTemplateOrTemplatePattern()));
       return true;
     }
     return RecursiveASTVisitor::TraverseTemplateArgumentLoc(TL);
   }
+
+  bool VisitCXXStdInitializerListExpr(CXXStdInitializerListExpr *E) {
+    // Reliance on initializer_lists requires std::initializer_list to be
+    // visible per standard. So report a reference to it, otherwise include of
+    // `<initializer_list>` might not receive any use.
+    report(E->getExprLoc(),
+           const_cast<CXXRecordDecl *>(E->getBestDynamicClassType()),
+           RefType::Implicit);
+    return true;
+  }
 };
 
 } // namespace
 
 void walkAST(Decl &Root, DeclCallback Callback) {
   ASTWalker(Callback).TraverseDecl(&Root);
 }
 
 } // namespace clang::include_cleaner

clang-tools-extra/include-cleaner/unittests/WalkASTTest.cpp

 namespace ns {
 template<class T> void $ambiguous^function(T);  // primary template
 template<> void $ambiguous^function(int);       // full specialization
 }
   )cpp",
            "using ns::^function;");
 }
 
-
 TEST(WalkAST, Alias) {
   testWalk(R"cpp(
     namespace ns { int x; }
     using ns::$explicit^x;
   )cpp",
            "int y = ^x;");
   testWalk("using $explicit^foo = int;", "^foo x;");
   testWalk("struct S {}; using $explicit^foo = S;", "^foo x;");

 }
 
 TEST(WalkAST, Enums) {
   testWalk("enum E { $explicit^A = 42, B = 43 };", "int e = ^A;");
   testWalk("enum class $explicit^E : int;", "enum class ^E : int {};");
   testWalk("enum class E : int {};", "enum class ^E : int ;");
 }
 
+TEST(WalkAST, InitializerList) {
+  testWalk(R"cpp(
+       namespace std {
+        template <typename T> struct $implicit^initializer_list {};
+       })cpp",
+           R"cpp(
+       const char* s = "";
+       auto sx = ^{s};)cpp");
+}
 } // namespace
 } // namespace clang::include_cleaner