[cDAC] efficient lookup of constructed types

[rcj1]

Currently the cDAC looks up constructed types as a linear search of the EEHashTable m_pAvailableTypeParams. This is not very efficient, so we will want to look at the performance and adjust the lookup mechanism accordingly.

Initial ideas

• Copy the runtime hash lookup algorithm
  -- Simplify the lookup algorithm in the runtime?
  -- If we choose this path, is there a way to minimize the complexity that we need in the cDAC?

• Store types in our own hash table
  -- Already doing this to some extent with a cache of already-found items
  -- Will have to deal with Flush()?

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