Implement glob import semantics for builtin type names

Currently we disallow any user-defined type name that collides with one of the builtin type names.

error[cxxbridge]: reserved name
  ┌─ src/main.rs:3:12
  │
3 │     struct UniquePtr { broken: bool }
  │            ^^^^^^^^^ reserved name

error[cxxbridge]: reserved name
  ┌─ src/main.rs:5:12
  │
5 │     struct c_char { broken: bool }
  │            ^^^^^^ reserved name

However, this is bad because it makes it a breaking change for us to add new builtin bindings, such as #681. If the user already had their own type with the same name as the new binding, that code needs to continue to work.

We need to implement basically the same semantics that Rust has for prelude types and glob imports — a user-defined type replaces any builtin binding of the same name.