Union type fixes and optimizations

[ahejlsberg]

This PR fixes issues relating to apparent types and union types and optimizes materialization of properties in union types. It also makes union types a separate kind of type, distinct from object types.

The following functions have changed:

Function	Change
getPropertiesOfType	Returns property list of any kind of type
getPropertyOfType	Returns an (apparent) property of any kind of type
getSignaturesOfType	Returns (apparent) call signatures of any kind of type
getIndexTypeOfType	Returns an (apparent) index signature of any kind of type
getPropertiesOfObjectType	Returns property list of an object type
getAugmentedPropertiesOfType	Returns augmented property list of any kind of type
getPropertyOfObjectType	Returns a property of an object type
getSignaturesOfObjectOrUnionType	Returns call signatures of an object or union type
getIndexTypeOfObjectOrUnionType	Returns index signature of an object or union type
resolveObjectOrUnionTypeMembers	Resolves members of object or union types

The functions in bold are the ones that code would typically use. They now automatically create synthetic results for union types and convert primitives and type parameters to their apparent types before performing the operation.

The ResolvedObjectType type has been renamed to ResolvedType because it now covers both object and union types. The ApparentType type is no longer needed and has been removed.

Materialization of union type properties previously happened in resolveObjectTypeMembers but is now done on demand per property through the getPropertyOfType function.