rustdoc where clause is not reproducible

[sanxiyn]

For example, run make doc/alloc/index.html twice in a row and check doc/alloc/boxed/struct.Boxed.html. Where clause of flat_map can be either where U: IntoIterator, F: FnMut(Self::Item) -> U, or where F: FnMut(Self::Item) -> U, U: IntoIterator.

I think it is suspect that where_cluases function in src/librustdoc/clean/simplify.rs uses HashMap.

[mitaa]

Related to #24473