[embedded] Add a frontend flag to drop all code from a module and emit an empty object file

[kubamracek]

Motivation: In Embedded Swift, we don't want to emit actual code for libraries, we only serialize them and clients only need the serialized .swiftmodule. But sometimes it's useful to still have some .o file for a library, because it simplifies the build system story significantly, and various build systems can still operate on a "normal" dependency graph. For that, let's add a flag that makes the compiler emit an empty object file, to be used in situations where a build system really needs some .o output.

I don't expect this flag to be useful when running manual compile commands (e.g. in a Makefile) -- users should simply avoid asking the compiler to generate a .o file in the first place.

Also note we should consider this to be a stop-gap, until we flesh out the story of how libraries/modules are supposed to build in Embedded Swift.

[kubamracek]

@swift-ci please test

[eeckstein]

lgtm

[etcwilde]

LGTM. Once the embedded stuff is stable and I have a minute, I can probably teach CMake how to model a build graph that doesn't have object files for Swift.