[clangd] Remove potential prefix from enum value names (#83412)

... when converting unscoped to scoped enums.
With traditional enums, a popular technique to guard against potential
name clashes is to use the enum name as a pseudo-namespace, like this:
  enum MyEnum { MyEnumValue1, MyEnumValue2 };
With scoped enums, this makes no sense, making it extremely unlikely
that the user wants to keep such a prefix when modernizing. Therefore,
our tweak now removes it.