[cxx-interop] Add `cassert` header to libstdc++ modulemap
[cxx-interop] Add `cassert` header to libstdc++ modulemap

Previously this header was sometimes getting hijacked by the first header to include it. This caused Swift compiler failures on Linux when using any LLVM header that uses `assert`.

Now `cassert` is referenced directly from a modulemap, which fixes the issue.

apple/swift,ahoppen/swift,benlangmuir/swift,apple/swift,roambotics/swift,apple/swift,glessard/swift,JGiola/swift,benlangmuir/swift,glessard/swift,roambotics/swift,roambotics/swift,JGiola/swift,benlangmuir/swift,glessard/swift,glessard/swift,roambotics/swift,apple/swift,ahoppen/swift,benlangmuir/swift,roambotics/swift,JGiola/swift,apple/swift,glessard/swift,JGiola/swift,roambotics/swift,ahoppen/swift,apple/swift,glessard/swift,JGiola/swift,ahoppen/swift,benlangmuir/swift,benlangmuir/swift,JGiola/swift,ahoppen/swift,ahoppen/swift