Fix single-module shim macro handling

[fallintoplace]

Summary

• Normalize ATOMICS_SINGLE_MODULE and SWIFTATOMIC_SINGLE_MODULE at the top of _AtomicsShims.h before either macro is used.
• Use ATOMICS_SINGLE_MODULE for the single-module _sa_retain_n and _sa_release_n declarations, matching the Xcode and Swift configuration name.
• Add a focused Xcode project smoke step that exercises the unmanaged reference path under the Atomics.xcodeproj scheme.

Why

Xcode defines ATOMICS_SINGLE_MODULE, and Swift checks ATOMICS_SINGLE_MODULE, but _AtomicsShims.h checked SWIFTATOMIC_SINGLE_MODULE for the calling-convention branch. That means the intended single-module declaration path could be skipped even though the rest of the Xcode build was in single-module mode.

Notes

The smoke step uses Xcode/Atomics.xcodeproj, since that is the project configuration that defines ATOMICS_SINGLE_MODULE through Xcode/Shared.xcconfig.