[include-cleaner] Ignore builtin symbols in the WalkAST.

[hokein]

Summary

There is no need to add headers for builtin symbols.

Additionally, there is a bonus benefit which help eliminate some bugs -- builtin
functions are modeled as implicit FunctionDecls in the clang AST, which results in
them being treated as normal FunctionDecls in the implementation of the include-cleaner
(going through the path: ast-node -> decl -> source location -> header).
And, the source location of these built-in symbols' AST nodes is not precise (e.g. points to the first call site),
which leads to subtle behavior that inserts a header of the call site.

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

 #include "clang/AST/DeclCXX.h"
 #include "clang/AST/Expr.h"
 #include "clang/AST/ExprCXX.h"
 #include "clang/AST/RecursiveASTVisitor.h"
 #include "clang/AST/TemplateBase.h"
 #include "clang/AST/TemplateName.h"
 #include "clang/AST/Type.h"
 #include "clang/AST/TypeLoc.h"
+#include "clang/Basic/IdentifierTable.h"
 #include "clang/Basic/SourceLocation.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/STLFunctionalExtras.h"
 #include "llvm/Support/Casting.h"
 
 namespace clang::include_cleaner {
 namespace {
 using DeclCallback =
     llvm::function_ref<void(SourceLocation, NamedDecl &, RefType)>;
 
 class ASTWalker : public RecursiveASTVisitor<ASTWalker> {
   DeclCallback Callback;
 
   void report(SourceLocation Loc, NamedDecl *ND,
               RefType RT = RefType::Explicit) {
     if (!ND || Loc.isInvalid())
       return;
+    // Don't report builtin symbols.
+    if (const auto *II = ND->getIdentifier(); II && II->getBuiltinID() > 0)
+      return;
     Callback(Loc, *cast<NamedDecl>(ND->getCanonicalDecl()), RT);
   }
 
   NamedDecl *resolveTemplateName(TemplateName TN) {
     // For using-templates, only mark the alias.
     if (auto *USD = TN.getAsUsingShadowDecl())
       return USD;
     return TN.getAsTemplateDecl();

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

 }
 
 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, BuiltinSymbols) {
+  testWalk(R"cpp(
+    extern "C" int __builtin_popcount(unsigned int) noexcept;
+  )cpp", "int x = ^__builtin_popcount(1);");
+}
+
 } // namespace
 } // namespace clang::include_cleaner