[clang-rename] Add the USR of incomplete decl to the USRSet.

[hokein]

Summary

This fixes a clangd rename issue, which is missing the reference of
an incomplete specialization.

Unfortunately, I didn't reproduce this issue in clang-rename, I guess
the input FoundDecl of AdditionalUSRFinder is different in clangd vs
clang-rename, clang-rename uses the underlying CXXRecordDecl of the
ClassTemplateDecl, which is fixed in https://github.com/llvm/llvm-project/commit/5d862c042b52ae2aad37471d0b83b6c678a520e3;
while clangd-rename uses the ClassTemplateDecl.

[kbobyrev]

Let me try to reproduce it in Clang-Rename. If I don't respond by tomorrow, feel free to submit the patch.

clang/lib/Tooling/Refactoring/Rename/USRFindingAction.cpp

130

Nit: const auto *Definition and RecordDecl could stay the same.

[kbobyrev]

Yeah, it works in Clang-Rename due to additional steps in the renaming process. Testing only Clangd's rename is sufficient here.

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

llvm::StringRef Tests[] = {
 
         void test() {
           [[Foo]]<int> x;
           [[Foo]]<bool> y;
           [[Foo]]<int*> z;
         }
       )cpp",
 
+      // Incomplete class specializations
+      R"cpp(
+        template <typename T>
+        class [[Fo^o]] {};
+        void func([[Foo]]<int>);
+      )cpp",
+
       // Template class instantiations.
       R"cpp(
         template <typename T>
         class [[F^oo]] {
         public:
           T foo(T arg, T& ref, T* ptr) {
             T value;
             int number = 42;

clang/lib/Tooling/Refactoring/Rename/USRFindingAction.cpp

void handleClassTemplateDecl(const ClassTemplateDecl *TemplateDecl) {
 
     for (const auto *PartialSpec : PartialSpecs) {
       if (PartialSpec->getSpecializedTemplate() == TemplateDecl)
         addUSRsOfCtorDtors(PartialSpec);
     }
     addUSRsOfCtorDtors(TemplateDecl->getTemplatedDecl());
   }
 
-  void addUSRsOfCtorDtors(const CXXRecordDecl *RecordDecl) {
-    RecordDecl = RecordDecl->getDefinition();
+  void addUSRsOfCtorDtors(const CXXRecordDecl *RD) {
+    const auto* RecordDecl = RD->getDefinition();
 
     // Skip if the CXXRecordDecl doesn't have definition.
-    if (!RecordDecl)
+    if (!RecordDecl) {
+      USRSet.insert(getUSRForDecl(RD));
       return;
+    }
 
     for (const auto *CtorDecl : RecordDecl->ctors())
       USRSet.insert(getUSRForDecl(CtorDecl));
     // Add template constructor decls, they are not in ctors() unfortunately.
     if (RecordDecl->hasUserDeclaredConstructor())
       for (const auto *D : RecordDecl->decls())
         if (const auto *FTD = dyn_cast<FunctionTemplateDecl>(D))
           if (const auto *Ctor =