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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@244912 91177308-0d34-0410-b5e6-96231b3b80d8