A copy constructor shouldn't prevent an automatic addition of a default instance constructor for a record. (#47850)

https://github.com/dotnet/csharplang/pull/3913

* Allow record's `parameter_list` to be empty.

Related to #47091
https://github.com/dotnet/csharplang/blob/master/meetings/2020/LDM-2020-08-24.md#base-calls-on-parameterless-records
https://github.com/dotnet/csharplang/pull/3900

Restrict readonly from being applied directly to init accessors, and prevent them from being considered readonly by the language.

* Use a distinct diagnostic ID when an exhaustiveness report uses an unnamed enum value.
Fixes #47066

Related to #47357.

* Decision dag: union the set of values when there are two predecessor states.

If a state has no set of values for a temp on one of the edges, that means it is unrestricted.
This fixes a bug in decision dag construction.  I was not able to construct a test that is
affected, but the change makes a visible difference in an internal dump of the decision dag
for the newly added test `MultiplePathsToState_01`, which we walked through in a compiler
team meeting on 2020-09-09.

* Use a builder as suggested in review.

This allows `init` properties on readonly structs, readonly properties in structs, and allows `init` accessors to be declared readonly themselves. Fixes https://github.com/dotnet/roslyn/issues/47612.

in the face of multiple when clauses, for the purpose of constructing the "not exhaustive" diagnostic.
Fixes #47164

Nameof's subexpressions do not require receivers, so this prevents us from having to handle that.
Fixes #46593

This threads through CancellationTokens to those parameters as long as
one was 'obviously' available for me to use; I didn't chase it too far
if it wasn't obvious to me.