[clangd] Find definition of ClassTemplate without going through index.

I noticed that, while go-to-def works on cases like:

namespace ns {
  template<typename T> struct Foo {};
}
using ::ns::Fo^o;

it only works because of the FileIndex. We can get definition location
directly from AST too.

Differential Revision: https://reviews.llvm.org/D113029

clang-tools-extra/clangd/XRefs.cpp

@@ -80,6 +80,9 @@ const NamedDecl *getDefinition(const NamedDecl *D) {
     return VD->getDefinition();
   if (const auto *FD = dyn_cast<FunctionDecl>(D))
     return FD->getDefinition();
+  if (const auto *CTD = dyn_cast<ClassTemplateDecl>(D))
+    if (const auto *RD = CTD->getTemplatedDecl())
+      return RD->getDefinition();
   // Objective-C classes can have three types of declarations:
   //
   // - forward declaration: @class MyClass;

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

@@ -675,7 +675,7 @@ TEST(LocateSymbol, All) {
 
       R"cpp(// Declaration of explicit template specialization
         template <typename T>
-        struct $decl[[Foo]] {};
+        struct $decl[[$def[[Foo]]]] {};
 
         template <>
         struct Fo^o<int> {};
@@ -683,12 +683,25 @@ TEST(LocateSymbol, All) {
 
       R"cpp(// Declaration of partial template specialization
         template <typename T>
-        struct $decl[[Foo]] {};
+        struct $decl[[$def[[Foo]]]] {};
 
         template <typename T>
         struct Fo^o<T*> {};
       )cpp",
 
+      R"cpp(// Definition on ClassTemplateDecl
+        namespace ns {
+          // Forward declaration.
+          template<typename T>
+          struct $decl[[Foo]];
+
+          template <typename T>
+          struct $def[[Foo]] {};
+        }
+
+        using ::ns::Fo^o;
+      )cpp",
+
       R"cpp(// auto builtin type (not supported)
         ^auto x = 42;
       )cpp",