Add tcRnGetNameToInstancesIndex

This function in tcRnDriver, retrieves an index by name of all Class and
Family instances in the current environment.

This is to be used by haddock which currently looks up instances for
each name, which looks at every instance for every lookup.

Using this function instead of tcRnGetInfo, the haddock.base performance
test improves by 10%

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: alexbiehl, rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3624