Pass-down -downgrade-typecheck-interface-error to interface verification job

The interface verification phase should downgrade errors to warnings as an
intermediate step to stage it in. This won't fail the build if interface errors
occur but it should help users verify project-side fixes.

Author: nkcsgexi

Sources/SwiftDriver/Jobs/VerifyModuleInterfaceJob.swift

@@ -26,6 +26,10 @@ extension Driver {
       outputs.append(TypedVirtualPath(file: outputPath, type: .diagnostics))
     }
 
+    // TODO: remove this because we'd like module interface errors to fail the build.
+    if isFrontendArgSupported(.downgradeTypecheckInterfaceError) {
+      commandLine.appendFlag(.downgradeTypecheckInterfaceError)
+    }
     return Job(
       moduleName: moduleOutputInfo.name,
       kind: .verifyModuleInterface,