rustdoc: use btree map for where clauses
to get more reproducible output.
Fixes: #32555
I've looked at the other uses of HashMap in rustdoc, and they seem ok to (i.e. they use `iter()` and related only for constructing a new map, or when the output goes into independent files).
Not sure what the cause of #24473 is, it shouldn't be where clauses, but maybe it was also fixed inbetween since May 2015.