Turns out that around 10.13/10.14 macOS release version, Apple changed the target tags in
tbd files from `macosx` to `macos`. In order to be compliant and therefore actually support
linking on older platforms against `libSystem.tbd`, we add `<cpu_arch>-macosx` to target strings.
This logic (currently) has a non-trivial cost (particularly in terms of
peak RSS) for tracking dependencies. Until incremental compilation is in
use in the wild, it doesn't make sense for users to pay that cost.
* Functions failing codegen now set this failure on the function
analysis state. Decl analysis `codegen_failure` is reserved for
failures generating constant values.
* `liveness_failure` is consolidated into `codegen_failure`, as we do
not need to distinguish these, and Liveness.Verify is just a debugging
feature anyway.
* `sema_failure_retryable` and `codegen_failure_retryable` are removed.
Instead, retryable failures are recorded in the new
`Zcu.retryable_failures` list. On an incremental update, this list is
flushed, and all elements are marked as outdated so that we re-attempt
analysis and code generation.
Also remove the `generation` fields from `Zcu` and `Decl` as these are
not needed by our new strategy for incremental updates.
* Mark root Decls for re-analysis separately
* Check for re-analysis of root Decls
* Remove `outdated` entry when analyzing fn body
* Remove legacy `outdated` field from Decl analysis state
* Invalidate `decl_val` dependencies
* Recursively mark and un-mark all dependencies correctly
* Queue analysis of outdated dependers in `Compilation.performAllTheWork`
Introduces logic to invalidate `decl_val` dependencies after
`Zcu.semaDecl` completes. Also, recursively un-mark dependencies as PO
where needed.
With this, all dependency invalidation logic is in place. The next step
is analyzing outdated dependencies and triggering appropriate
re-analysis.
Sema now tracks dependencies appropriately. Early logic in Zcu for
resolving outdated decls/functions is in place. The setup used does not
support `usingnamespace`; compilations using this construct are not yet
supported by this incremental compilation model.
