[FR] Bionic support for module maps

I recently tried building the llbuild C++ project on Android in the Termux app with the Swift package manager, which adds `-fmodules` by default. While compiling llbuild like this works on linux with glibc and macOS, it doesn't work on Android (with errors like [the size_t one explained in the clang docs](https://releases.llvm.org/9.0.0/tools/clang/docs/Modules.html#modularizing-a-platform)) or Windows, so I submitted a pull to remove that flag on Android for now, apple/swift-package-manager@00c7768a.

Are there any plans to get the Bionic headers working with that flag? If so, maybe this can be the tracking issue till it's in.

Start with the Bionic headers and the linked Clang module documentation, then reproduce the llbuild C++ build on Android with -fmodules enabled. Done means the Bionic headers work with that flag without the reported size_t and related errors.

Written by the indexing model from the issue text.