Added the _modules workaround to make the compiler pull in the correct
module when someone does `import stdc_assert` (for instance).
Fix various niggles.
rdar://123503615
We need to declare separate top-level modules for every header in
the musl C library. Note that our set-up here does require some
tweaks to musl itself (specifically, the `alltypes.h` header needs
some fairly serious massaging to make it module compatible; in
particular, we need to pull out every type declaration into its
own separate modularized header in `bits/types`).
Also update `Platform.swift` a little for more musl support.
rdar://123503615
