Parameter default values should be accessible from bound nodes and/or symbols

[RikkiGibson]

Related to #47171 (comment)

Basically, the "true" default values for optional parameters depend on a variety of factors, including the call site and attributes used on the parameter. Many of these factors aren't resolved until lowering of a call. It feels like nullable analysis among other things could be more accurate if the following changes were made:

1. A BoundCall should contain child nodes for the implicitly passed default arguments. The constant values of those default arguments are the same as what we will produce during emit.
2. A BoundMethodBody should contain child nodes for parameters with default values. This part of the design has been dropped. See https://github.com/dotnet/csharplang/blob/master/proposals/nullable-parameter-default-value-analysis.md

/cc @333fred