perf(compiler): only emit changed files for incremental compilation

For now, we always create all generated files, but diff them
before we pass them to TypeScript.

For the user files, we compare the programs and only emit changed
TypeScript files.

This also adds more diagnostic messages if the `—diagnostics` flag
is passed to the command line.

packages/compiler-cli/test/transformers/compiler_host_spec.ts

@@ -51,7 +51,7 @@ describe('NgCompilerHost', () => {
   } = {}) {
     return new TsCompilerAotCompilerTypeCheckHostAdapter(
         ['/tmp/index.ts'], options, ngHost, new MetadataCollector(), codeGenerator,
-        librarySummaries);
+        new Map(librarySummaries.map(entry => [entry.fileName, entry] as[string, LibrarySummary])));
   }
 
   describe('fileNameToModuleName', () => {