Created by: robbertvanginkel
This pr adds the functionality to support modular mixed objective-c/swift librariesusing the -import-underlying-module flag. The new swift metadata type SWIFT_UNDERLYING_MODULE refers to a build rule for a headersymlinktree that exposes the public Objective-C headers through a module with the same name, so that the swift compiler can read this using the import-underlying-module flag when compiling the swift sources for the library.
Based on having support for modular libraries in #1874.