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)

[333fred]

Done review pass (commit 7)

[333fred]

Done review pass. Just a couple of small things.

[cston]

                parameterDefaultValueBuilder.Add(constVal);

Consider moving this after the if (constVal != null) { ... } check, essentially moving a version of the if below here, to avoid needing the comment.

if (constVal != null && parameterDefaultValueBuilder == null)
{
    parameterDefaultValueBuilder = ... // allocate and fill
}
if (parameterDefaultValueBuilder != null)
{
    parameterDefaultValueBuilder.Add(constVal);
}

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In reply to: 1213551892

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Refers to: src/Compilers/CSharp/Portable/BoundTree/UnboundLambda.cs:664 in 8254fca. [](commit_id = 8254fca, deletion_comment = False)

[333fred]

Done review pass. Overall looking good, but test failure needs to be addressed.

[333fred]

LGTM (commit 21), assuming tests are passing.