[clangd] AddUsing: do not crash on non-namespace using decls.

[hokein]

thanks!

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

for (auto &U : Usings) {
     // Only "upgrade" to fully qualified is all relevant using decls are fully
     // qualified. Otherwise trust what the user typed.
     if (!isFullyQualified(U->getQualifier()))
       AlwaysFullyQualify = false;
 
     if (SM.isBeforeInTranslationUnit(Inputs.Cursor, U->getUsingLoc()))
       // "Usings" is sorted, so we're done.
       break;
-    if (U->getQualifier()->getAsNamespace()->getCanonicalDecl() ==
+    if (const auto *Namespace = U->getQualifier()->getAsNamespace()) {
+      if (Namespace->getCanonicalDecl() ==
               QualifierToRemove.getNestedNameSpecifier()
                   ->getAsNamespace()
                   ->getCanonicalDecl() &&
           U->getName() == Name) {
         return InsertionPointData();
       }
+    }
 
     // Insertion point will be before last UsingDecl that affects cursor
     // position. For most cases this should stick with the local convention of
     // add using inside or outside namespace.
     LastUsingLoc = U->getUsingLoc();
   }
   if (LastUsingLoc.isValid() && IsValidPoint(LastUsingLoc)) {
     InsertionPointData Out;

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

namespace xx {
   void yy();
 }
 
 using xx::yy;
 
 void fun() {
   yy();
 }
+)cpp"},
+            // Existing using with non-namespace part.
+            {R"cpp(
+#include "test.hpp"
+using one::two::ee::ee_one;
+one::t^wo::cc c;
+)cpp",
+             R"cpp(
+#include "test.hpp"
+using one::two::cc;using one::two::ee::ee_one;
+cc c;
 )cpp"}};
   llvm::StringMap<std::string> EditedFiles;
   for (const auto &Case : Cases) {
     for (const auto &SubCase : expandCases(Case.TestSource)) {
       ExtraFiles["test.hpp"] = R"cpp(
 namespace one {
 void oo() {}
 namespace two {
-enum ee {};
+enum ee {ee_one};
 void ff() {}
 class cc {
 public:
   struct st { struct nested {}; };
   static void mm() {}
 };
 }
 using uu = two::cc;