Swift doesn’t recognize Clang’s header on Windows #60534

Describe the bug
Clang provides some C headers as polyfills (or shims) and the Swift toolchain vendors it in /usr/lib/clang/{clang_version}/include. However, the Swift compiler doesn’t recognize these headers, so a module produced by the bundled Clang may not be consumed by Swift.

Steps To Reproduce
Steps to reproduce the behavior:

1. Clone https://github.com/apple/swift-atomics;
2. Change directory to swift-atomics;
3. Run swift build.

Expected behavior
Since the _AtomicsShims target is successfully compiled, it should be consumable for Atomics.

Screenshots

Environment

• OS: Windows Server 2019 Datacenter (1809)
• Swift version: 5.6.2
• Visual Studio: Community 2019, 16.11.18

Additional context
Although this can be worked around from the SwiftPM side, I believe resolving it with swiftc is a better choice.