CMake Embedded Stdlib: mark ExecutorImpl.cpp as optional

This is needed for internal Apple configurations that build the Embedded
Stdlib on its own.

Addresses rdar://149410405

Author: edymtt

stdlib/public/Concurrency/CMakeLists.txt

@@ -265,6 +265,7 @@ if(SWIFT_SHOULD_BUILD_EMBEDDED_STDLIB AND SWIFT_SHOULD_BUILD_EMBEDDED_CONCURRENC
   set(SWIFT_STDLIB_CONCURRENCY_TRACING FALSE)
   set(SWIFT_STDLIB_HAS_ENVIRON FALSE)
   set(SWIFT_STDLIB_HAS_ASL FALSE)
+  list(APPEND LLVM_OPTIONAL_SOURCES ExecutorImpl.cpp)
 
   foreach(entry ${EMBEDDED_STDLIB_TARGET_TRIPLES})
     string(REGEX REPLACE "[ \t]+" ";" list "${entry}")