Auto merge of #32244 - Amanieu:compare_exchange_result, r=alexcrichton

Change compare_exchange to return a Result<T, T>

As per the discussion in #31767

I also changed the feature name from `extended_compare_and_swap` to `compare_exchange`.

r? @alexcrichton

diff --cc src/libcore/intrinsics.rs
Simple merge

diff --cc src/librustc_trans/trans/intrinsic.rs
Simple merge

diff --cc src/librustc_typeck/check/intrinsic.rs
Simple merge