Automatically copy Swift.h header for static libraries

[brentleyjones]

Resolves #350, addressing 2.i.

SWIFT_OBJC_INTERFACE_HEADER_NAME is set by Xcode correctly depending on if you have Swift sources or not. We can't see that though and always get nil for it unless it's overridden. Because of this we have to check our Compile Source files for any Swift files. We only do so after ensuring that we are looking at a static library that hasn't set SWIFT_OBJC_INTERFACE_HEADER_NAME to "", for the best performance.

I initially went the route of having these files be copied with a Copy Files phase, but it had some ugliness:

• The files used "${CONFIGURATION_TEMP_DIR}" in their path, which means you couldn't open them and they were always red.
• The path wasn't 100% correct, because some variables could be changed by Xcode and we wouldn't know where the file would be located.

[brentleyjones]

@keith Wanted to make sure this is a no-op for you. The SWIFT_OBJC_INTERFACE_HEADER_NAME != "" check should prevent it from doing anything in your case.

[keith]

No diff, thanks!

[brentleyjones]

@yonaskolb This is the last thing needed to complete #350. Let me know if there is anything I can do to get this in 😺

[yonaskolb]

Looks good. Could we add a unit test and something in the TestProject fixture though?