Add missing compat libraries to bootstrapping

The bootstrapping stages were missing dependencies on the Concurrency
and Swift 5.6 backdeployment compatibility libraries resulting in
failing builds.

Author: etcwilde

SwiftCompilerSources/CMakeLists.txt

@@ -295,6 +295,8 @@ else()
       set(compatibility_libs
           "swiftCompatibility50-${platform}"
           "swiftCompatibility51-${platform}"
+          "swiftCompatibility56-${platform}"
+          "swiftCompatibilityConcurrency-${platform}"
           "swiftCompatibilityDynamicReplacements-${platform}")
 
       list(APPEND b0_deps ${compatibility_libs})