[Modules] Add Darwin-specific compatibility module map parsing hacks

This preserves backwards compatibility for two hacks in the Darwin
system module map files:

1. The use of 'requires excluded' to make headers non-modular, which
should really be mapped to 'textual' now that we have this feature.

2. Silently removes a bogus cplusplus requirement from IOKit.avc.

Once we start diagnosing missing requirements and headers on
auto-imports these would have broken compatibility with existing Darwin
SDKs.

llvm-svn: 244912