[clang-tidy] Fix a crash issue on misc-virtual-near-miss check.

[hokein]

Summary

The crash is caused by triggering a Assertion failed in DeclCXX.h when the
check detects non-defined class return type in a class method declaration.

[congliu]

LGTM. Thanks!

clang-tidy/misc/VirtualNearMissCheck.cpp

static bool checkOverridingFunctionReturnType(const ASTContext *Context,
   QualType DTy = DerivedReturnTy->getPointeeType().getCanonicalType();
   QualType BTy = BaseReturnTy->getPointeeType().getCanonicalType();
 
   const CXXRecordDecl *DRD = DTy->getAsCXXRecordDecl();
   const CXXRecordDecl *BRD = BTy->getAsCXXRecordDecl();
   if (DRD == nullptr || BRD == nullptr)
     return false;
 
+  if (!DRD->hasDefinition() || !BRD->hasDefinition())
+    return false;
+
   if (DRD == BRD)
     return true;
 
   if (!Context->hasSameUnqualifiedType(DTy, BTy)) {
     // Begin checking whether the conversion from D to B is valid.
     CXXBasePaths Paths(/*FindAmbiguities=*/true, /*RecordPaths=*/true,
                        /*DetectVirtual=*/false);
 

test/clang-tidy/misc-virtual-near-miss.cpp

 // RUN: %check_clang_tidy %s misc-virtual-near-miss %t
 
+class NoDefinedClass1;
+class NoDefinedClass2;
+
 struct Base {
   virtual void func();
   virtual void gunk();
   virtual ~Base();
   virtual Base &operator=(const Base &);
+  virtual NoDefinedClass1 *f();
 };
 
 struct Derived : Base {
   // Should not warn "do you want to override 'gunk'?", because gunk is already
   // overriden by this class.
   virtual void funk();
   // CHECK-MESSAGES: :[[@LINE-1]]:3: warning: method 'Derived::funk' has a similar name and the same signature as virtual method 'Base::func'; did you mean to override it? [misc-virtual-near-miss]
 
   void func2();
   // CHECK-MESSAGES: :[[@LINE-1]]:3: warning: method 'Derived::func2' has {{.*}} 'Base::func'
 
   void func22(); // Should not warn.
 
   void gunk(); // Should not warn: gunk is override.
 
   void fun();
   // CHECK-MESSAGES: :[[@LINE-1]]:3: warning: method 'Derived::fun' has {{.*}} 'Base::func'
 
   Derived &operator==(const Base &); // Should not warn: operators are ignored.
+
+  virtual NoDefinedClass2 *f1(); // Should not crash: non-defined class return type is ignored.
 };
 
 typedef Derived derived_type;
 
 class Father {
 public:
   Father();
   virtual void func();