[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.

tmp vfs

Author: egorzhdan

stdlib/public/Cxx/libstdcxx.modulemap

@@ -20,4 +20,13 @@ module std {
   header "libstdcxx.h"
   requires cplusplus
   export *
+
+  /// C compatibility headers.
+  module compat {
+    module cassert {
+      header "cassert"
+      requires cplusplus
+      export *
+    }
+  }
 }