[clangd] Fix AddUsing tweak for out-of-line functions.

Summary:
We used getEnclosingNamespaceContext(), which calls getParent() rather
than getLexicalParent(), so we would end up adding the "using" line in
places that do not affect the cursor location, or just return an error
when declaration was in another file.

Patch by Adam Czachorowski!

Reviewers: hokein

Reviewed By: hokein

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D79496

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

@@ -141,10 +141,12 @@ findInsertionPoint(const Tweak::Selection &Inputs,
   }
 
   // No relevant "using" statements. Try the nearest namespace level.
-  const auto *NS = Inputs.ASTSelection.commonAncestor()
-                       ->getDeclContext()
-                       .getEnclosingNamespaceContext();
-  if (auto *ND = dyn_cast<NamespaceDecl>(NS)) {
+  const DeclContext *ParentDeclCtx =
+      &Inputs.ASTSelection.commonAncestor()->getDeclContext();
+  while (ParentDeclCtx && !ParentDeclCtx->isFileContext()) {
+    ParentDeclCtx = ParentDeclCtx->getLexicalParent();
+  }
+  if (auto *ND = llvm::dyn_cast_or_null<NamespaceDecl>(ParentDeclCtx)) {
     auto Toks = Inputs.AST->getTokens().expandedTokens(ND->getSourceRange());
     const auto *Tok = llvm::find_if(Toks, [](const syntax::Token &Tok) {
       return Tok.kind() == tok::l_brace;

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

@@ -2663,6 +2663,23 @@ using one::two::ff;
 
 void fun() {
   CALL(ff);
+})cpp"},
+            // Parent namespace != lexical parent namespace
+            {R"cpp(
+#include "test.hpp"
+namespace foo { void fun(); }
+
+void foo::fun() {
+  one::two::f^f();
+})cpp",
+             R"cpp(
+#include "test.hpp"
+using one::two::ff;
+
+namespace foo { void fun(); }
+
+void foo::fun() {
+  ff();
 })cpp"}};
   llvm::StringMap<std::string> EditedFiles;
   for (const auto &Case : Cases) {