AstGen: add result location analysis pass

The main motivation for this change is eliminating the `block_ptr`
result location and corresponding `store_to_block_ptr` ZIR instruction.
This is achieved through a simple pass over the AST before AstGen which
determines, for AST nodes which have a choice on whether to provide a
result location, which choice to make, based on whether the result
pointer is consumed non-trivially.

This eliminates so much logic from AstGen that we almost break even on
line count! AstGen no longer has to worry about instruction rewriting
based on whether or not a result location was consumed: it always knows
what to do ahead of time, which simplifies a *lot* of logic. This also
incidentally fixes a few random AstGen bugs related to result location
handling, leading to the changes in `test/` and `lib/std/`.

This opens the door to future RLS improvements by making them much
easier to implement correctly, and fixes many bugs. Most ZIR is made
more compact after this commit, mainly due to not having redundant
`store_to_block_ptr` instructions lying around, but also due to a few
bugs in the old system which are implicitly fixed here.

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