feat(@angular-devkit/build-optimizer): add support for es2015 enums e…

[alan-agius4]

…mitted by tsickle

tsickle emits es2015 enums with an object literal followed by an export declaration

Example:

      const RendererStyleFlags3 = {
          Important: 1,
          DashCase: 2,
      };
      export { RendererStyleFlags3 };
      RendererStyleFlags3[RendererStyleFlags3.Important] = 'Important';
      RendererStyleFlags3[RendererStyleFlags3.DashCase] = 'DashCase';

This PR adds support for the enums to be optimized by wrapping them in an iife and marks them as pure.

Fixes #13488

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