[include-cleaner] Respect the UsingShadowDecl when find headers for ambiguous std symbols. (#66485)

In libcpp, the `std::remove(const char*)` is a using decl in std
namespace `using ::remove`, which was not handled correctly in
`headerForAmbiguousStdSymbol`

Author: hokein

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

@@ -13,6 +13,7 @@
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/Decl.h"
 #include "clang/AST/DeclBase.h"
+#include "clang/AST/DeclCXX.h"
 #include "clang/Basic/Builtins.h"
 #include "clang/Basic/FileEntry.h"
 #include "clang/Basic/SourceLocation.h"
@@ -116,6 +117,8 @@ std::optional<tooling::stdlib::Header>
 headerForAmbiguousStdSymbol(const NamedDecl *ND) {
   if (!ND->isInStdNamespace())
     return {};
+  if (auto* USD = llvm::dyn_cast<UsingShadowDecl>(ND))
+    ND = USD->getTargetDecl();
   const auto *FD = ND->getAsFunction();
   if (!FD)
     return std::nullopt;

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

@@ -11,6 +11,7 @@
 #include "clang-include-cleaner/Analysis.h"
 #include "clang-include-cleaner/Record.h"
 #include "clang-include-cleaner/Types.h"
+#include "clang/AST/Expr.h"
 #include "clang/AST/RecursiveASTVisitor.h"
 #include "clang/Basic/FileEntry.h"
 #include "clang/Basic/FileManager.h"
@@ -587,6 +588,36 @@ TEST_F(HeadersForSymbolTest, AmbiguousStdSymbols) {
   }
 }
 
+TEST_F(HeadersForSymbolTest, AmbiguousStdSymbolsUsingShadow) {
+  Inputs.Code = R"cpp(
+    void remove(char*);
+    namespace std { using ::remove; }
+
+    void k() {
+      std::remove("abc");
+    }
+  )cpp";
+  buildAST();
+
+  // Find the DeclRefExpr in the std::remove("abc") function call.
+  struct Visitor : public RecursiveASTVisitor<Visitor> {
+    const DeclRefExpr *Out = nullptr;
+    bool VisitDeclRefExpr(const DeclRefExpr *DRE) {
+      EXPECT_TRUE(Out == nullptr) << "Found multiple DeclRefExpr!";
+      Out = DRE;
+      return true;
+    }
+  };
+  Visitor V;
+  V.TraverseDecl(AST->context().getTranslationUnitDecl());
+  ASSERT_TRUE(V.Out) << "Couldn't find a DeclRefExpr!";
+  EXPECT_THAT(headersForSymbol(*(V.Out->getFoundDecl()),
+                               AST->sourceManager(), &PI),
+              UnorderedElementsAre(
+                  Header(*tooling::stdlib::Header::named("<cstdio>"))));
+}
+
+
 TEST_F(HeadersForSymbolTest, StandardHeaders) {
   Inputs.Code = "void assert();";
   buildAST();