[clang-tidy] Exclude forward decls from fuchsia-multiple-inheritance

[juliehockett]

Summary

Addresses b39770.

clang-tools-extra/trunk/clang-tidy/fuchsia/MultipleInheritanceCheck.cpp

 }
 
 void MultipleInheritanceCheck::registerMatchers(MatchFinder *Finder) {
   // Requires C++.
   if (!getLangOpts().CPlusPlus)
     return;
 
   // Match declarations which have bases.
-  Finder->addMatcher(cxxRecordDecl(hasBases()).bind("decl"), this);
+  Finder->addMatcher(
+      cxxRecordDecl(allOf(hasBases(), isDefinition())).bind("decl"), this);
 }
 
 void MultipleInheritanceCheck::check(const MatchFinder::MatchResult &Result) {
   if (const auto *D = Result.Nodes.getNodeAs<CXXRecordDecl>("decl")) {
     // Check against map to see if if the class inherits from multiple
     // concrete classes
     unsigned NumConcrete = 0;
     for (const auto &I : D->bases()) {

clang-tools-extra/trunk/test/clang-tidy/fuchsia-multiple-inheritance.cpp

public:
   int val = 0;
 };
 
 class Interface_with_A_Parent : public Base_A {
 public:
   virtual int baz() = 0;
 };
 
+// Shouldn't warn on forward declarations.
+class Bad_Child1;
+
 // Inherits from multiple concrete classes.
 // CHECK-MESSAGES: [[@LINE+2]]:1: warning: inheriting mulitple classes that aren't pure virtual is discouraged [fuchsia-multiple-inheritance]
 // CHECK-NEXT: class Bad_Child1 : public Base_A, Base_B {};
 class Bad_Child1 : public Base_A, Base_B {};
 
 // CHECK-MESSAGES: [[@LINE+1]]:1: warning: inheriting mulitple classes that aren't pure virtual is discouraged [fuchsia-multiple-inheritance]
 class Bad_Child2 : public Base_A, Interface_A_with_member {
   virtual int foo() override { return 0; }