Define UB in float-to-int casts to saturate

- Round to zero, and representable values cast directly.
- `NaN` goes to 0
- Values beyond the limits of the type are saturated to the "nearest value"
  (essentially rounding to zero, in some sense) in the integral type, so e.g.
  `f32::INFINITY` would go to `{u,i}N::MAX.`

src/librustc_interface/tests.rs

@@ -558,7 +558,7 @@ fn test_debugging_options_tracking_hash() {
     tracked!(sanitizer, Some(Sanitizer::Address));
     tracked!(sanitizer_memory_track_origins, 2);
     tracked!(sanitizer_recover, vec![Sanitizer::Address]);
-    tracked!(saturating_float_casts, true);
+    tracked!(saturating_float_casts, Some(true));
     tracked!(share_generics, Some(true));
     tracked!(show_span, Some(String::from("abc")));
     tracked!(src_hash_algorithm, Some(SourceFileHashAlgorithm::Sha1));