Instead of computing signatures during building, compute them afterwards for verification (#51718)

This helps with finding issues with d.ts emit because of caching.

src/compiler/sys.ts

@@ -1433,7 +1433,6 @@ export interface System {
 
     // For testing
     /** @internal */ now?(): Date;
-    /** @internal */ disableUseFileVersionAsSignature?: boolean;
     /** @internal */ storeFilesChangingSignatureDuringEmit?: boolean;
 }