Initialize long return type descs in `gtNewCallNode`.

Initialize long return type descs in `gtNewCallNode`.

All long-returning calls (including helper calls) must have a properly
initialized ReturnTypeDesc. Previously, this ReturnTypeDesc was only
initialized by the importer for a subset of long-returning calls
(essentially user calls), which caused asserts whenother long-returning
calls were created by the compiler (e.g. for field accessors). This
change moves the necessary ReturnTypeDesc initialization into
`gtNewCallNode`.