In `ASTWriter`, input files are sorted based on whether they are system or user. The current implementation used single `std::queue` with `push_back` and `push_front`. This resulted in the user files being reversed.
This patch fixes that by keeping the system/user distinction, but otherwise serializing files in the order they were loaded by the `SourceManager`. This is then used in the dependency scanner to report module map dependencies in the correct order.
Depends on D134224.
Reviewed By: Bigcheese
Differential Revision: https://reviews.llvm.org/D134248
Jan Svoboda