Update CMake supported clang versions

[JJTech0130]

• Darling doesn't build with clang versions less than 9, so there is no need to support them: NSObject.o build error: prefix attribute must be followed by an interface or protocol #939 and error: prefix attribute must be followed by an interface or protocol #964. EDIT: Fixed by 4fbf800.
• Clang 11 is easier to get than 9 or 10 (buster-backports only has 8 and 11) so we should add it for ease of use.
• Clang 12 support seems to be resolved: TARGET_OS_DRIVERKIT missing under Clang 12 #856.

I also updated the docs to reflect this change: darlinghq/darling-docs#25

[bugaevc]

To be honest, I'd like to keep old versions of Clang working too. If something we wrote doesn't work with them, I'd consider it a bug. However, if it's something Apple wrote (like objc4), then there's not much we could do.

[JJTech0130]

I mentioned some issues in darlinghq/darling-docs#25, and they were closed with "your clang is too old".
EDIT: Updated description with links to mentioned issues.

[bugaevc]

So objc_nonlazy_class looks like it can be trivially made to work on older Clang versions:

#if /* have new enough Clang */
__attribute__((objc_nonlazy_class))
#endif
@implementation Foo

#if /* have old Clang */
+ (void) load { }
#endif

(Empty load methods for NSObject and others is also what the older versions of objc4 use.)

cc @facekapow

[facekapow]

Yeah, that look about right (not sure if that's all that older clangs would need, though). That's actually what OS_OBJECT_NONLAZY_CLASS and OS_OBJECT_NONLAZY_CLASS_LOAD in libdispatch do. The objc runtime would be the only one that needs to be updated (everything else uses libdispatch's macros).

[JJTech0130]

Well, um, I think that's ok. We should still add the new versions. Can I remove commits from the pull request, or is that not possible?

[facekapow]

Yes; you can reset your branch back to master, and then make new commits, and then force push to the branch.

[JJTech0130]

Well, can't do that on mobile, and my pc is a bit stressed out trying to build darling... Will do it as soon as I finish.

[facekapow]

Currently testing that potential fix for older clangs. I'm on Arch, though, so I can't test Clang 6 specifically (well, I could, but the AUR only has Clang 5 and then 7), only Clang 7.

[JJTech0130]

OK. I have fixed the commits.

[JJTech0130]

How should the docs be updated? The Debian 10 requirements specified clang-6.0, whilst there is a notice stating only clang 9+ is supported. Should the notice be changed? Removed? And should we leave the Debian 10 instructions or update them?

[facekapow]

The docs should be updated to indicate that the minimum required Clang version is actually 6 now, not 9.

[JJTech0130]

darlinghq/darling-docs#26
I closed the other pull req. as there is no need to update the clang dep. for Debian 10 anymore.