fix(compiler): recover from structural errors in watch mode (#19953)

This also changes the compiler so that we throw less often
on structural changes and produce a meaningful state
in the `ng.Program` in case of errors.

Related to #19951

PR Close #19953

packages/compiler-cli/src/perform_watch.ts

@@ -191,6 +191,10 @@ export function performWatchCompilation(host: PerformWatchHost):
       };
     }
     ingoreFilesForWatch.clear();
+    const oldProgram = cachedProgram;
+    // We clear out the `cachedProgram` here as a
+    // program can only be used as `oldProgram` 1x
+    cachedProgram = undefined;
     const compileResult = performCompilation({
       rootNames: cachedOptions.rootNames,
       options: cachedOptions.options,