Add flag for building target module graph with experimental feature Embedded

[rauhul]

Description

Instead of manually tagging every target you want to build with Embedded, we should add a flag to do this from swift build.

This flag should work in concert with the existing --target and --product options to filter the build graph.

Something like:

$ swift build --product Foo --enable-experimental-feature-Embedded

I propose we do not add a general mechanism for --enable-experimental-feature at this time, as it is unclear to me if that is desirable.

[MaxDesiatov]

@bnbarham do you think this could be generalized not just for embedded?

[rauhul]

FWIW we could leave the door more open by parsing --enable-experimental-feature X but only allowing X = Embedded for now

[rauhul]

This should now be done with a toolset instead of a top-level SwiftPM build flag. Embedded is a property of a target platform.

[MaxDesiatov]

This should now be done with a toolset instead of a top-level SwiftPM build flag. Embedded is a property of a target platform.

How do you suggest handling empty object files? These can't be handled for the whole object graph in a toolset and still need to be conditionally added per target. Closing this issue seems premature before that problem is resolved.

[rauhul]

This should now be done with a toolset instead of a top-level SwiftPM build flag. Embedded is a property of a target platform.

How do you suggest handling empty object files? These can't be handled for the whole object graph in a toolset and still need to be conditionally added per target. Closing this issue seems premature before that problem is resolved.

#8654 should address this