Improve CSystem import handling

[jrflat]

System previously imported CSystem with a bare (public) import. As a result, building with library evolution (e.g. as an OS framework) added import CSystem to System's .swiftinterface, which then failed to verify with "no such module 'CSystem'". On Darwin, _O_CLOFORK's inlinable body also referenced a macro defined in CSystem.

Switching to internal imports alone doesn't work: CSystemLinux.h re-included glibc headers, which made CSystem a second owner of declarations like EPERM and mode_t. These declarations would then be treated as internal and couldn't be used inside inlinable code. Now:

• CSystemLinux.h no longer re-includes libc headers that Glibc exports. Those declarations are now attributed to the libc module.
• Every CSystem import is internal, except Constants.swift on WASI and FreeBSD, whose inlinable constants still come from CSystem.
• IORing's io_uring types move to a new CSystemIOUring module, which the IORing files import publicly. The syscall wrappers move into shims.c.
• Darwin's _O_CLOFORK is now the literal 0x0800_0000, since older SDKs lack O_CLOFORK, and CSystemDarwin.h is removed. A new test checks the literal against the SDK's O_CLOFORK.
• Windows files import WinSDK directly, and CSystemWindows.h is removed.
• CInterop.swift imports Android instead of Bionic, like the other files.

[jrflat]

2 breaking changes detected in SystemPackage:
  💔 API breakage: accessor IORing.RegisteredResource.unsafeBuffer.Get() has generic signature change from <T where T == CSystem.iovec> to <T where T == SwiftGlibc.iovec>
  💔 API breakage: accessor IORing.RegisteredResource.bytes.Get() has generic signature change from <T where T == CSystem.iovec> to <T where T == SwiftGlibc.iovec>

This is expected since CSystem.iovec is now attributed as SwiftGlibc.iovec. The actual symbol should still be compiled as __C.iovec. A client who calls import CSystem (even though it's not a product) to name iovec without import Glibc would now need import Glibc. I highly doubt this case exists.

Once we merge, the CI failure will go away.