cmake: avoiding generating ABI baseline files when building stdlib

We have a regression test checked in the compiler repository to detect ABI breakges as the
stdlib evolves, therefore we don't need to generate these side car files for ABI checking
purposes that could be potentially done externally.

Related to: rdar://118465899

Author: nkcsgexi

stdlib/cmake/modules/SwiftSource.cmake

@@ -583,6 +583,11 @@ function(_compile_swift_files
     list(APPEND swift_flags "-swift-version" "5")
   endif()
 
+  # Avoiding emiting ABI descriptor files while building stdlib.
+  if (SWIFTFILE_IS_STDLIB)
+    list(APPEND swift_flags "-Xfrontend" "-empty-abi-descriptor")
+  endif()
+
   if(SWIFTFILE_IS_SDK_OVERLAY)
     list(APPEND swift_flags "-autolink-force-load")
   endif()