Declare SwiftCore_CONCURRENCY_GLOBAL_EXECUTOR

We need to define the defaulted setting so that it pulls in the default
value. Otherwise we hit a configuration failure in
`Concurrency/CMakeLists.txt` when it includes the config file specific
to the global executor because `SwiftCore_CONCURRENCY_GLOBAL_EXECUTOR`
is an empty string.

Author: etcwilde

Runtimes/Core/CMakeLists.txt

@@ -112,6 +112,7 @@ defaulted_option(SwiftCore_ENABLE_COMPACT_ABSOLUTE_FUNCTION_POINTERS "Resolve ab
 defaulted_option(SwiftCore_ENABLE_BACKDEPLOYMENT_SUPPORT "Add symbols for runtime backdeployment")
 defaulted_option(SwiftCore_ENABLE_STDLIB_TRACING "Enable tracing in the runtime. Assumes the presence of os_log(3) and the os_signpost(3) API.")
 defaulted_option(SwiftCore_ENABLE_CONCURRENCY "Enable Concurrency runtime support")
+defaulted_set(SwiftCore_CONCURRENCY_GLOBAL_EXECUTOR STRING "Default Concurrency global executor implementation")
 option(SwiftCore_ENABLE_UNICODE_DATA "Include unicode data in Swift runtimes" ON)
 option(SwiftCore_ENABLE_SHORT_MANGLING_LOOKUPS "Build with fast-path context descriptor lookups based on well-known short manglings." ON)
 option(SwiftCore_ENABLE_FILESYSTEM_SUPPORT "Build for systems that have a filesystem" ON)