Rollup merge of #106468 - compiler-errors:err-instability, r=lcnr

Use FxIndexSet when updating obligation causes in `adjust_fulfillment_errors_for_expr_obligation`

I have no idea how to test this reliably, but I've **manually** verified it fixes the instability in #106417 that isn't due to dtolnay/trybuild#212.

Fixes #106417

diff --cc compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs
Simple merge