Including select builtin headers in system modules is a workaround for module cycles, primarily in Apple's Darwin module that includes all of its C standard library headers. The workaround is problematic because it doesn't include all of the builtin headers (inttypes.h is notably absent), and it also doesn't include C++ headers. The straightforward for for this is to make top level modules for all of the C standard library headers and unwind.h in C++, clang, and the OS.

However, doing so in clang before the OS modules are ready re-introduces the module cycles. Add a -fbuiltin-headers-in-system-modules option to control if the special builtin headers belong to system modules or builtin modules. Pass the option by default for Apple.

Reviewed By: ChuanqiXu, Bigcheese, benlangmuir

Differential Revision: https://reviews.llvm.org/D159483

(cherry picked from commit 0ea3d88)

__has_feature(builtin_headers_in_system_modules) was added in
https://reviews.llvm.org/D159483 to be used in the stdarg/stddef
implementation headers. It ended up being unnecessary, but I forgot to
remove the feature definition.

(cherry picked from commit 6d18951)

…ses (llvm#102239)

The upcoming Apple SDK releases will support the clang built-in headers
being in the clang built-in modules: stop passing
-fbuiltin-headers-in-system-modules for those SDK versions.

(cherry picked from commit 9616399)

…atalyst (llvm#104872)

Mac Catalyst is the iOS platform, but it builds against the macOS SDK
and so it needs to be checking the macOS SDK version instead of the iOS
one. Add tests against a greater-than SDK version just to make sure this
works beyond the initially supporting SDKs.

(cherry picked from commit b986438)