Validate CRDs before generating TypeDefs

[knottnt]

Issue #, if available:

Description of changes:
The model.GetTypeDefs function became very expensive if validation of CRDs would fail. The root cause being that the IsShapeUsedInCRDs() check in GetTypeDefs() attempts to build the set of CRDs and doesn't return an error if the CRDs have a validation error. When this happens returns false and the shape is skipped. However, since IsShapeUsedInCRDs() failed to lazy load the set of CRDs each shape invokes IsShapeUsedInCRDs and attempt to rebuild all CRDs. For large AWS model files this can become very expensive and take many minutes before reporting the error.

This PR addresses the issue by explicitly evaluating CRDs before calling GetTypeDefs to fail fast on CRD validation errors.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[a-hilaly]

/lgtm

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: a-hilaly, knottnt

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@knottnt: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
verify-attribution	0ca161a	link	false	/test verify-attribution
s3-controller-test	0ca161a	link	unknown	/test s3-controller-test
ec2-controller-test	0ca161a	link	unknown	/test ec2-controller-test
dynamodb-controller-test	0ca161a	link	unknown	/test dynamodb-controller-test

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