Integrate Lambda Default Parameters into Synthesized Delegate Types and Lowering Pass

[adamperlin]

Allows default values to be attached to the parameters of synthesized delegate types and threaded through the closure lowering pass in order to support code-gen for lambda default parameters.

Still missing (will be added in a subsequent PR):

• Diagnostics for invalid target-type conversions between lambdas with default parameters and delegate types
• Synthesizing delegate types for method groups with default parameters (and associated conversion warnings)