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
`ObjCRuntime` and `CommentOpts.BlockCommandNames` are checked by
`ASTReader::checkLanguageOptions`, but are not part of the module
context hash. This can lead to errors when using implicit modules if
different TUs have different values for these options when using the
same module cache.
This was not hit very often due to the rare usage of
`-fblock-command-names=` and that `ObjCRuntime` is by default set by
the target triple, which is part of the existing context hash.
