Clarify ambiguity in select_nth_unstable docs
Original docs for `select_nth_unstable` family of functions were ambiguous as to whether "the element at `index`" was the element at `index` before the function reordered the elements or after the function reordered the elements.
The most helpful change in this PR is to change the given examples to make this absolutely clear. Before, "the element at `index`" was the same value before and after the reordering, so it didn't help disambiguate the meaning. I've changed the example for `select_nth_unstable` and `select_nth_unstable_by` so that "the element at `index`" is different before and after the reordering, which clears up the ambiguity. The function `select_nth_unstable_by_key` already had an example that was unambiguous.
In an attempt to clear up the ambiguity from the get-go, I've added a bit of redundancy to the text. Now the docs refer to "the element at `index` *after the reordering*".
Move predicate, region, and const stuff into their own modules in middle
This PR mostly moves things around, and in a few cases adds some `ty::` to the beginning of names to avoid one-off imports.
I don't mean this to be the most *thorough* move/refactor. I just generally wanted to begin to split up `ty/mod.rs` and `ty/sty.rs` which are huge and hard to distinguish, and have a lot of non-ty stuff in them.
r? lcnr
CI: Use ninja on apple builders
This switches the apple builders to use ninja when building LLVM. My hope is that this should resolve the timeouts we have been experiencing since December. My theory is that something in the image update from [Dec 20](dec20a5272) is causing an issue with our build (or, perhaps more remotely, some [update to LLVM itself](https://github.com/rust-lang/rust/commits/master/src/llvm-project)).
The symptoms are that during the LLVM build it just hangs just before the install step. The last thing it prints is `[100%] Built target llvm-reduce` and then just hangs. Normally the next part should be `Install the project...` where it starts installing LLVM. I'm able to reproduce this without too much difficulty. I've been testing ninja, and it seems to be working better (however, my test isn't quite equivalent, since I'm getting sccache misses, and I can't update the S3 bucket).
Installing ninja takes about 7 to 10 seconds, so it shouldn't impact things. I can't determine if it will affect the overall build timing due to not being able to test with a warm S3 cache.
