perf(compiler): speed up watch mode (#19275)

- don’t regenerate code for .d.ts files when
  an oldProgram is passed to `createProgram`
- cache `fileExists` / `getSourceFile` / `readFile` in watch mode
- refactor tests to share common code in `test_support`
- support `—diagnostic` command line to print total time
  used per watch mode compilation.
PR Close #19275

packages/compiler-cli/src/ngtools_api2.ts

@@ -66,8 +66,9 @@ export interface CompilerHost extends ts.CompilerHost {
 export enum EmitFlags {
   DTS = 1 << 0,
   JS = 1 << 1,
+  Codegen = 1 << 4,
 
-  Default = DTS | JS
+  Default = DTS | JS | Codegen
 }
 
 export interface CustomTransformers {
@@ -106,6 +107,7 @@ export interface Program {
     customTransformers?: CustomTransformers,
     emitCallback?: TsEmitCallback
   }): ts.EmitResult;
+  getLibrarySummaries(): {fileName: string, content: string}[];
 }
 
 // Wrapper for createProgram.