[clangd] Fix a crash in DefineInline

clang-tools-extra/clangd/refactor/tweaks/DefineInline.cpp

 // Returns the canonical declaration for the given FunctionDecl. This will
 // usually be the first declaration in current translation unit with the
 // exception of template specialization.
 // For those we return first declaration different than the canonical one.
 // Because canonical declaration points to template decl instead of
 // specialization.
 const FunctionDecl *findTarget(const FunctionDecl *FD) {
   auto CanonDecl = FD->getCanonicalDecl();
-  if (!FD->isFunctionTemplateSpecialization())
+  if (!FD->isFunctionTemplateSpecialization() || CanonDecl == FD)
     return CanonDecl;
   // For specializations CanonicalDecl is the TemplatedDecl, which is not the
   // target we want to inline into. Instead we traverse previous decls to find
   // the first forward decl for this specialization.
   auto PrevDecl = FD;
   while (PrevDecl->getPreviousDecl() != CanonDecl) {
     PrevDecl = PrevDecl->getPreviousDecl();
     assert(PrevDecl && "Found specialization without template decl");

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

TEST_F(DefineInlineTest, TemplateSpec) {
   EXPECT_AVAILABLE(R"cpp(
     template <typename T> void foo();
     void bar();
     template <> void foo<int>();
 
     template<> void f^oo<int>() {
       bar();
     })cpp");
+  EXPECT_UNAVAILABLE(R"cpp(
+    namespace bar {
+      template <typename T> void f^oo() {}
+      template void foo<int>();
+    })cpp");
 }
 
 TEST_F(DefineInlineTest, CheckForCanonDecl) {
   EXPECT_UNAVAILABLE(R"cpp(
     void foo();
 
     void bar() {}
     void f^oo() {