fix(@angular/build): skip semantic diagnostics for declaration files

Declaration files don't produce semantic or template diagnostics, so
calling getSemanticDiagnostics() on them was just wasted work. Move
the isDeclarationFile guard to before that call so .d.ts files are
skipped entirely.

Author: arturovt

packages/angular/build/src/tools/angular/compilation/aot-compilation.ts

@@ -234,6 +234,7 @@ export class AotCompilation extends AngularCompilation {
 
     const syntactic = modes & DiagnosticModes.Syntactic;
     const semantic = modes & DiagnosticModes.Semantic;
+    const skipLibCheck = typeScriptProgram.getCompilerOptions().skipLibCheck;
 
     // Collect program level diagnostics
     if (modes & DiagnosticModes.Option) {
@@ -265,13 +266,19 @@ export class AotCompilation extends AngularCompilation {
         continue;
       }
 
+      // When skipLibCheck is enabled (the default), declaration files are not
+      // type-checked, so getSemanticDiagnostics() would return nothing for them.
+      if (sourceFile.isDeclarationFile && skipLibCheck) {
+        continue;
+      }
+
       yield* profileSync(
         'NG_DIAGNOSTICS_SEMANTIC',
         () => typeScriptProgram.getSemanticDiagnostics(sourceFile),
         true,
       );
 
-      // Declaration files cannot have template diagnostics
+      // Declaration files cannot have template diagnostics.
       if (sourceFile.isDeclarationFile) {
         continue;
       }