[AST] Consider QualifiedTemplateName in TemplateName::getAsUsingDecl().

If the underlying template name of a qualified template name is a using
decl, TemplateName::getAsUsingDecl() will return it.

This will make the UsingTemplateName consumer life easier.

Differential Revision: https://reviews.llvm.org/D124437

clang-tools-extra/clangd/unittests/IncludeCleanerTests.cpp

@@ -90,6 +90,14 @@ TEST(IncludeCleaner, ReferencedLocations) {
           template <template <typename> class T> class X {};
           X<A> x;
         )cpp"},
+      {R"cpp(
+          namespace ns { template<typename T> class A {}; }
+          namespace absl {using ns::^A;}
+       )cpp",
+       R"cpp(
+          template <template <typename> class T> class X {};
+          X<absl::A> x;
+       )cpp"},
       {R"cpp(
           namespace ns { template<typename T> struct ^A { ^A(T); }; }
           using ns::^A;

clang/lib/AST/TemplateName.cpp

@@ -172,6 +172,8 @@ UsingShadowDecl *TemplateName::getAsUsingShadowDecl() const {
   if (Decl *D = Storage.dyn_cast<Decl *>())
     if (UsingShadowDecl *USD = dyn_cast<UsingShadowDecl>(D))
       return USD;
+  if (QualifiedTemplateName *QTN = getAsQualifiedTemplateName())
+    return QTN->getUnderlyingTemplate().getAsUsingShadowDecl();
   return nullptr;
 }
 

clang/unittests/AST/TemplateNameTest.cpp

@@ -82,6 +82,7 @@ TEST(TemplateName, QualifiedUsingTemplate) {
   const auto *USD = QTN->getUnderlyingTemplate().getAsUsingShadowDecl();
   EXPECT_TRUE(USD);
   EXPECT_EQ(USD->getTargetDecl(), TN.getAsTemplateDecl());
+  EXPECT_EQ(TN.getAsUsingShadowDecl(), USD);
 }
 
 TEST(TemplateName, UsingTemplate) {