* add `Module.setBlockBody` and related functions
* redo astgen for `and` and `or` to use fewer ZIR instructions and
require less processing for comptime known values
* Sema: rework `analyzeBody` function. See the new doc comments in this
commit. Divides ZIR instructions up into 3 categories:
- always noreturn
- never noreturn
- sometimes noreturn
The current plan is to avoid using async and related features in the
stage2 compiler so that we can bootstrap before implementing them.
Having this untested and incomplete code in the codebase increases
friction while working on stage2, in particular when preforming
larger refactors such as the current zir memory layout rework.
Therefore remove all async related code, leaving only error messages
in astgen.
These were previously implemented as a sub/sub_wrap instruction with a
lhs of 0. Making this separate instructions however allows us to save
some memory as there is no need to store a lhs.
* free Module.Fn ZIR code when destroying the owner Decl
* unreachable_safe and unreachable_unsafe are collapsed into one ZIR
instruction with a safety flag.
* astgen: emit an unreachable instruction for unreachable literals
* don't forget to call deinit on ZIR code
* astgen: implement some builtin functions
We are now passing this test:
```zig
export fn _start() noreturn {}
```
```
test.zig:1:30: error: expected noreturn, found void
```
I ran into an issue where we get an integer overflow trying to compute
node index offsets from the containing Decl. The problem is that the
parser adds the Decl node after adding the child nodes. For some things,
it is easy to reserve the node index and then set it later, however, for
this case, it is not a trivial code change, because depending on tokens
after parsing the decl determines whether we want to add a new node or
not.
Possible strategies here:
1. Rework the parser code to make sure that Decl nodes are before
children nodes in the AST node array.
2. Use signed integers for Decl node offsets.
3. Just flip the order of subtraction and addition. Expect Decl Node
index to be greater than children Node indexes.
I opted for (3) because it seems like the simplest thing to do. We'll
want to unify the logic for computing the offsets though because if the
logic gets repeated, it will probably get repeated wrong.
There are some `@panic("TODO")` in there but I'm trying to get the
branch to the point where collaborators can jump in.
Next is to repair the seam between LazySrcLoc and codegen's expected
absolute file offsets.
Next up is reworking the seam between the LazySrcLoc emitted by Sema
and the byte offsets currently expected by codegen.
And then the big one: updating astgen.zig to use the new memory layout.
