Skip default CMO when emitting module separately

Cross-module optimization is incompatible with emitting the module
separately. Previously, an assertion caught this condition, but the
assertion would not fire in release builds.

This changes the logic to simply skip enabling default CMO when
emitModuleSeparately is true, rather than asserting or erroring.

Author: rjmansfield

Sources/SwiftDriver/Jobs/CompileJob.swift

@@ -296,12 +296,14 @@ extension Driver {
                                  forObject: outputType == .object)
     try addRuntimeLibraryFlags(commandLine: &commandLine)
 
+    // Enable default cross-module optimization when possible, but only if we're not
+    // emitting the module separately (CMO is incompatible with separate module emission)
     if Driver.canDoCrossModuleOptimization(parsedOptions: &parsedOptions) &&
        // For historical reasons, -cross-module-optimization turns on "aggressive" CMO
        // which is different from "default" CMO.
        !parsedOptions.hasArgument(.CrossModuleOptimization) &&
-       !parsedOptions.hasArgument(.EnableCMOEverything) {
-      assert(!emitModuleSeparately, "Cannot emit module separately with cross-module-optimization")
+       !parsedOptions.hasArgument(.EnableCMOEverything) &&
+       !emitModuleSeparately {
       commandLine.appendFlag("-enable-default-cmo")
     }