This ensures only one process is generating module cache from an interface
file so that we don't blow up memory usage when multiple processes are
doing the same. The locking mechanism is similar to that of Clang's.
A better approach is that the build system takes care of the module building
step as a formal dependency.
rdar://52839445
This change ensures using .swiftmodule built from source has the same behavior as
using .swiftmodule built from .swiftinterface.
A swift-ide-test utility is added to print linked libraries from a Swift module for
testing purposes.
rdar://58057556
