Produce unreachable_patterns warning when deserialization names collide

[dtolnay]

Closes #2308.

warning: unreachable pattern
 --> src/main.rs:7:22
  |
5 |     #[serde(rename = "Response")]
  |                      ---------- matches all the relevant values
6 |     Request { id: String},
7 |     #[serde(rename = "Response")]
  |                      ^^^^^^^^^^ no value can reach this
  |
  = note: `#[warn(unreachable_patterns)]` on by default