Assign E0784 to union expression error

compiler/rustc_error_codes/src/error_codes/E0784.md

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+A union expression does not have exactly one field.
+
+Erroneous code example:
+
+```compile_fail,E0784
+union Bird {
+    pigeon: u8,
+    turtledove: u16,
+}
+
+let bird = Bird {}; // error
+let bird = Bird { pigeon: 0, turtledove: 1 }; // error
+```
+
+The key property of unions is that all fields of a union share common storage.
+As a result, writes to one field of a union can overwrite its other fields, and
+size of a union is determined by the size of its largest field.
+
+You can find more information about the union types in the [Rust reference].
+
+Working example:
+
+```
+union Bird {
+    pigeon: u8,
+    turtledove: u16,
+}
+
+let bird = Bird { pigeon: 0 }; // OK
+```
+
+[Rust reference]: https://doc.rust-lang.org/reference/items/unions.html