[clangd][NFX][FIX] Fix conflicting symbol name Expr

Summary:
This class has a member named `Expr` which conflicts with the clang type
of the same name due to the namespace. If we want to do this we need to
explicitly scope the variable. This was preventing me from building.
This was introduced in https://reviews.llvm.org/D148457

clang-tools-extra/clangd/Hover.cpp

@@ -468,7 +468,7 @@ struct PrintExprResult {
   std::optional<std::string> PrintedValue;
   /// The Expr object that represents the closest evaluable
   /// expression.
-  const Expr *Expr;
+  const clang::Expr *Expr;
   /// The node of selection tree where the traversal stops.
   const SelectionTree::Node *Node;
 };