Compute FunctionTypeImpl type parameters lazily.
Compute FunctionTypeImpl type parameters lazily.

This will allow a FunctionTypeImpl to be constructed without having to
consult the underlying element, which is necessary to allow the
element model to be reconstituted from a summary.

R=brianwilkerson@google.com

Review URL: https://codereview.chromium.org/1557213002 .

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