build: treat SourceKit as a host tool

[compnerd]

This changes the SourceKit build to use the host configuration. This is
the appropriate way to handle the build of SourceKit as it is not a
target component. Doing so simplifies the build, makes it more obvious
how to migrate the SourceKit build to the standard CMake build
techniques.

Replace this paragraph with a description of your changes and rationale. Provide links to external references/discussions if appropriate.

Resolves SR-NNNN.

[compnerd]

@swift-ci please clean test

[compnerd]

CC: @Rostepher @edymtt @xiaobai @akyrtzi

[akyrtzi]

\cc @benlangmuir

We have internal clients that cross-compile SourceKit for iOS, though I'm unfamiliar with how exactly they configure. Will these changes prevent SourceKit from being buildable for iOS?
Or is this similar to how the compiler binary itself is configured so cross-compilation will work the same?

[compnerd]

Yeah, its the same way that the compiler cross-compiles itself that you would do to build it for iOS. Are you building SourceKit or the swift-lang bits for iOS btw? Because that has a whole other set of complexity :(

[akyrtzi]

Just the C++ SourceKit code.

[benlangmuir]

The principle here seems correct to me. Do you have any suggestions for seeing what exactly will change in practice? Configure with ninja and then dump the commands I guess?

[compnerd]

Yeah, that would be the most effective way to identify the practical change.

[shahmishal]

Please update the base branch to main by Oct 5th otherwise the pull request will be closed automatically.

• How to change the base branch: (Link)
• More detail about the branch update: (Link)