Commits on Oct 1, 2024

1. Annotate _FoundationCShims as a system module …

   This fixes Clang modularization errors coming out of the Swift compiler when building Swift code that uses C++ interoperability on Linux:
   ```
   /usr/include/c++/v1/istream:198:5: error: 'std::basic_istream<wchar_t>::basic_istream' from module 'SwiftGlibc' is not present in definition of 'std::wistream' in module '_FoundationCShims'
    196 | protected:
    197 |     inline _LIBCPP_INLINE_VISIBILITY
    198 |     basic_istream(basic_istream&& __rhs);
   ```
   
   Two observations:
   
   1. _FoundationCShims does not list all the headers that it includes in the modulemap. Instead, the modulemap refers to a single headers with several `#include` directives. This means that a different module that includes the same headers might hijack them from _FoundationCShims, depending on compiler flags and the order of inclusion in the translation unit.
   
   2. Some of the headers that _FoundationCShims includes are C headers that have corresponding C++ compatibility headers. When C++ interop is enabled, the C++ headers will be included. Those headers are owned by the std module.
   
   Annotating the _FoundationCShims module as a system module suppresses the errors about these.
   
   rdar://137044915

   

   egorzhdan committed Oct 1, 2024
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