Improve swift dependency scanner by validating and selecting dependency
module into scanner. This provides benefits that:
* Build system does not need to schedule interface compilation task if
the candidate module is picked, it can just use the candidate module
directly.
* There is no need for forwarding module in the explicit module build.
Since the build system is coordinating the build, there is no need for
the forwarding module in the module cache to avoid duplicated work,
* This also correctly supports all the module loading modes in the
dependency scanner.
This is achieved by only adding validate and up-to-date binary module as
the candidate module for swift interface module dependency. This allows
caching build to construct the correct dependency in the CAS. If there
is a candidate module for the interface module, dependency scanner will
return a binary module dependency in the dependency graph.
The legacy behavior is mostly preserved with a hidden frontend flag
`-no-scanner-module-validation`, while the scanner output is mostly
interchangeable with new scanner behavior with `prefer-interface` module
loading mode except the candidate module will not be returned.
rdar://123711823
Model indexing output as an optional output from the swift compiler
as the build system has no knowledge about them and they can be
regenerated by indexer. Make sure the indexing store output is produced
when cache hit so the compilation is done for the module. If cache hit,
no indexing data is produced since no compilation is done.
rdar://123331335
