Instead of replacing an interface file with its up-to-date compile module,
the dep-scanner should report potentially up-to-date module candidates either adjacent to
the interface file or in the prebuilt module cache. swift-driver should later pass down
these candidates to -compile-module-from-interface invocation and the front-end job
will check if one of the candidates is ready to use. The front-end job then either emits a forwarding
module to an up-to-date candidate or a binary module.
For the explicit module mode, swift-driver uses -compile-module-from-interface to
generate modules from interfaces found by the dependency scanner. However, we don't
need to build the binary module if up-to-date modules are available, either adjacent
to the interface file or in the prebuilt module cache directory. This patch teaches
dependencies scanner to report these ready-to-use binary modules.
Building each Swift module explicitly requires dependency PCMs to be built
with the exactly same deployment target version. This means we may need to
build a Clang module multiple times with different target triples.
This patch removes the -target arguments from the reported PCM build
arguments and inserts extraPcmArgs fields to each Swift module.
swift-driver can combine the generic PCM arguments with these extra arguments
to get the command suitable for building a PCM specifically for
that loading Swift module.
