Sema: rewrite peer type resolution

The existing logic for peer type resolution was quite convoluted and
buggy. This rewrite makes it much more resilient, readable, and
extensible. The algorithm works by first iterating over the types to
select a "strategy", then applying that strategy, possibly applying peer
resolution recursively.

The new semantics around PTR for comptime-known fixed-width integers did
require some small changes to parts of the compiler, std, and
compiler_rt. Regarding the MachO changes, I spoke to @kubkon about how
best to do them, and this is what he suggested.

Several new tests have been added to cover cases which the old logic did
not correctly handle.

Resolves: #15644
Resolves: #15693
Resolves: #15709