fix(ngcc): handle aliases in UMD export declarations (#38959)

Sometimes UMD exports appear in the following form:

```
exports.MyClass = alias1 = alias2 = <<declaration>>
```

Previously the declaration of the export would have been captured
as `alias1 = alias2 = <<declaration>>`, which the `PartialInterpreter`
would have failed on, since it cannot handle assignments.

Now we skip over these aliases capturing only the `<<declaration>>`
expression.

Fixes #38947

PR Close #38959

packages/compiler-cli/ngcc/test/host/umd_host_spec.ts

@@ -638,7 +638,8 @@ runInEachFileSystem(() => {
               `  exports.d = b;\n` +
               `  exports.e = e;\n` +
               `  exports.DirectiveX = core.Directive;\n` +
-              `  exports.SomeClass = SomeClass;\n` +
+              `  var SomeClass_1;\n` +
+              `  exports.SomeClass = SomeClass_1 = SomeClass;\n` +
               `})));\n`,
         },
         {