Update with new warnings about xcconfigs HOT 4 CLOSED

From the release notes, these also look interesting:
• -Wframework-include-private-from-public
• -Watimport-in-framework-header

I haven‘t yet looked up the config names of these yet.

from xcconfigs.

The use of @import in framework headers prevent headers being used without modules. A new diagnostic detects the use of @import in framework headers when you pass the -fmodules flag. The diagnostic is OFF by default in clang and is controlled using the -Watimport-in-framework-header flag. (39192894)

from xcconfigs.

Warns when an atomic is used with an implicitly sequentially-consistent memory order, instead of explicitly specifying memory order.
CLANG_WARN_ATOMIC_IMPLICIT_SEQ_CST

from xcconfigs.

@fabb

-Wframework-include-private-from-public
Seems to be on by default, and I couldn't find it in Xcode.

-Watimport-in-framework-header
I also couldn't find this one in Xcode.

While we could enable them via the WARNING_CFLAGS, it's harder for users to disable them on the per project basis. I'd skip them for now.

from xcconfigs.