[CMake] Improve Xcode project generation with folders

[jrose-apple]

• Re-enable the use of folders with the USE_FOLDER setting. This got lost a while ago when we stopped including LLVM's top-level CMakeLists.txt.

• Put a bunch of new targets into folders.

Should not affect the built product and definitely shouldn't affect anyone not building with Xcode (or MSVC, I guess).

[jrose-apple]

@swift-ci Please smoke test

[gottesmm]

@jrose-apple is there a way that we can test this?

[jrose-apple]

I don't really see one, no. I don't want to write a test poking at the generated Xcode's project structure, and simply asserting that the CMake targets have such a property is more of a step than we want to take at the moment. (That is, there are targets without a FOLDER and I don't want to track them down right now; these were just the biggest offenders.)

[gottesmm]

@jrose-apple that isn't what I was suggesting. What I was suggesting was a build system unit test that uses:

xcodebuild -scheme "schemeName" -showBuildSettings

for one of these and maybe grep to see if it has that property. That way we can be sure that this does not break in the future.

[gottesmm]

that being said, given that we do not run a bot with Xcode, perhaps such a test is not interesting (since nothing will prevent it from breaking). Your thoughts?

[jrose-apple]

I'd be open to doing such a thing but I don't know of any supported/stable way to do it. This isn't a build setting; it's "just" the structure in the navigator.

[gottesmm]

I understand that. I was asking b/c you said that this used to work b/c we were importing more of LLVM's cmake (I guess). Whenever something like this breaks, I ask this question. That being said, I think listing/dumping/grepping the settings would work. That being said, I am not going to argue strongly for this.

[Rostepher]

I'm fine with these CMake changes. 👍