[clangd] Make PopulateSwitch a fix.

[sammccall]

Summary

It fixes the -Wswitch warning, though we mark it as a fix even if that is off.
This makes it the "recommended" action on an incomplete switch, which seems OK.

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

 namespace clangd {
 namespace {
 class PopulateSwitch : public Tweak {
   const char *id() const override;
   bool prepare(const Selection &Sel) override;
   Expected<Effect> apply(const Selection &Sel) override;
   std::string title() const override { return "Populate switch"; }
   llvm::StringLiteral kind() const override {
-    return CodeAction::REFACTOR_KIND;
+    return CodeAction::QUICKFIX_KIND;
   }
 
 private:
   const DeclContext *DeclCtx = nullptr;
   const SwitchStmt *Switch = nullptr;
   const CompoundStmt *Body = nullptr;
   const EnumType *EnumT = nullptr;
   const EnumDecl *EnumD = nullptr;