fix(core): property overrides sometimes don't work with intrinsics

[corymhall]

When you add a property override, we merge it with any properties set
when the construct is initialized. The merge happens after tokens are
resolved. For example, if we created a resource:

const resource = new CfnResource(this, 'Resource', {
  type: 'MyResourceType',
  properties: {
    prop1: Fn.ref('Param'),
  },
});

And the added an override for prop1:

resource.addPropertyOverride('prop1', Fn.ref('OtherParam'));

We would then perform a deep merge on the properties with a priority on
the overrides. The above example would work fine, eventually the merge
would get to the point where it was comparing two objects:

target = { prop1: { Ref: 'Param' } }
source = { prop1: { Ref: 'OtherParam' } }
result = { prop1: { Ref: 'OtherParam' } }

If we instead added an override using a different intrinsic:

resource.addPropertyOverride('prop1', Fn.join('-', ['hello',
Fn.ref('Param')]));

Then we would get to the point in the merge where we were comparing two
different objects:

target = { prop1: { Ref: 'Param' } }
source = { prop1: { 'Fn::Join': ['-', ['hello', { Ref: 'Param' } ]] } }
result = { prop1: {
  Ref: 'Param',
  'Fn::Join': ['-', ['hello', { Ref: 'Param' } ]] },
}}

This PR solves this issue by filtering out any CloudFormation
intrinsics. If the merge finds an intrinsic key in the target it "drops" the
intrinsic and takes whatever value is in the source (override).

fix #19971, #19447

---

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[gitpod-io]

[Naumel]

[nitpick] Are the { and [ reversed from L#347 to L#553?

[mergify]

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[aws-cdk-automation]

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[mergify]

Thank you for contributing! Your pull request will be updated from main and then merged automatically (do not update manually, and be sure to allow changes to be pushed to your fork).