[CMake] Include apinotes in StandaloneOverlay.cmake (#21928)

...because the Darwin overlay is responsible for installing them.

Context: Within Apple, overlays are sometimes built separately from
the rest of the Swift project. This doesn't currently work with just
the public repo (rdar://problem/47324189), but it could, and I broke
it with my recent API notes cleanup. This should get things working
again.

rdar://problem/47294352

Author: jrose-apple

cmake/modules/StandaloneOverlay.cmake

@@ -61,6 +61,7 @@ precondition(TOOLCHAIN_DIR)
 # Some overlays include the runtime's headers,
 # and some of those headers are generated at build time.
 add_subdirectory("${SWIFT_SOURCE_DIR}/include" "swift/include")
+add_subdirectory("${SWIFT_SOURCE_DIR}/apinotes" "swift/apinotes")
 
 # Without this line, installing components is broken. This needs refactoring.
 swift_configure_components()