The language service incorrectly reports an error if it fails to find
NgModule metadata for a particular Component / Directive. In many cases,
the use case is legit, particularly in test.
This commit removes such diagnostic message and cleans up the interface
for `TypeScriptHost.getTemplateAst()`.
PR closes https://github.com/angular/vscode-ng-language-service/issues/463
PR Close#34113