The code of `ScanDependencies.cpp` was creating invalid JSON since #66031
because in the case of having `extraPcmArgs` and `swiftOverlayDependencies`,
but not `bridgingHeader`, a comma will not be added at the end of
`extraPcmArgs`, creating an invalid JSON file. Additionally that same PR
added a trailing comma at the end of the `swiftOverlayDependencies`, which
valid JSON does not allow, but that bug was removed in #66366.
Both problems are, however, present in the 5.9 branch, because #66936
included #66031, but not #66366.
Besides fixing the problem in `ScanDependencies.cpp` I modified every test
that uses `--scan-dependencies` to pass the produced JSON through
Python's `json.tool` in order to validate proper JSON is produced. In
most cases I was able to pipe the output of the tool into `FileCheck`,
but in some cases the validation is done by itself because the checks
depend on the exact format generated by `--scan-dependencies`. In
a couple of tests I added a call to `FileCheck` that seemed to be
missing.
Without these changes, two tests seems to be generating invalid JSON in
my machine:
- `ScanDependencies/local_cache_consistency.swift` (which outputs `Expecting ',' delimiter: line 525 column 11 (char 22799)`)
- `ScanDependencies/placholder_overlay_deps.swift`
This change causes the cache to be layered with a local "cache" that wraps the global cache, which will serve as the source of truth. The local cache persists only for the duration of a given scanning action, and has a store of references to dependencies resolved as a part of the current scanning action only, while the global cache is the one that persists across scanning actions (e.g. in `DependencyScanningTool`) and stores actual module dependency info values.
Only the local cache can answer dependency lookup queries, checking current scanning action results first, before falling back to querying the global cache, with queries disambiguated by the current scannning action's search paths, ensuring we never resolve a dependency lookup query with a module info that could not be found in the current action's search paths.
This change is required because search-path disambiguation can lead to false-negatives: for example, the Clang dependency scanner may find modules relative to the compiler's path that are not on the compiler's direct search paths. While such false-negative query responses should be functionally safe, we rely on the current scanning action's results being always-present-in-the-cache for the scanner's functionality. This layering ensures that the cache use-sites remain unchanged and that we get both: preserved global state which can be queried disambiguated with the search path details, and an always-consistent local (current action) cache state.
