This commit also reclaims +2 ZIR instruction tags by moving the
following to `extended`:
* func_var_args
* func_extra
* func_extra_var_args
The following ZIR instruction tag is added:
* func_inferred
* AstGen: represent compile errors in ZIR rather than returning
`error.AnalysisFail`.
* ZIR: remove decl_ref and decl_val instructions. These are replaced by
`decl_ref_named` and `decl_val_named`, respectively, which will
probably get renamed in the future to the instructions that were just
deleted.
* AstGen: implement `@This()`, `@fence()`, `@returnAddress()`, and
`@src()`.
* AstGen: struct_decl improved to support fields_len=0 but have decls.
* AstGen: fix missing null bytes after compile error messages.
* SrcLoc: no longer depend on `Decl`. Instead have an explicit field
`parent_decl_node` which is an absolute AST Node index.
* Module: `failed_files` table can have null value, in which case the
key, which is a `*Scope.File`, will have ZIR errors in it.
* ZIR: implement text rendering of struct decls.
* CLI: introduce debug_usage and `zig astgen` command which is enabled
when the compiler is built in debug mode.
See #8516.
* AstGen is now done on whole files at once rather than per Decl.
* Introduce a new wait group for AstGen tasks. `performAllTheWork`
waits for all AstGen tasks to be complete before doing Sema,
single-threaded.
- The C object compilation tasks are moved to be spawned after
AstGen, since they only need to complete by the end of
the function.
With this commit, the codebase compiles, but much more reworking is
needed to get things back into a useful state.
