Auto-default ref fields with a null ref-assignment

[jcouv]

Vlad mentioned that the fixed statement can null-out references. Found that code in LocalRewriter_FixedStatement.cs and factored it out into a NullRef helper.
This is emitted using the same IL as ref T Unsafe.NullRef<T>(), namely ldc.i4.0 followed by conv.u.

No spec update required, as I think the desired behavior is already covered by auto-default-structs.md:

1. When the this variable itself does not meet the requirements, then all unassigned instance variables within this at all points where requirements are violated are implicitly initialized to the default value (§9.3) in an initialization phase before any other code in the constructor runs.
2. When an instance variable v within this does not meet the requirements, or any instance variable at any level of nesting within v does not meet the requirements, then v is implicitly initialized to the default value in an initialization phase before any other code in the constructor runs.

Fixes #63018

Relates to test plan #59194

[cston]

InitRefField_AutoDefault()

Consider moving above ScopedReserved_* tests, to avoid a conflict with #62783. #Resolved

[jcouv]

@cston FYI, I skipped one test following integration with runtime flag change (that test needs to use Net60 framework and use the test hook...).

[jcouv]

@dotnet/roslyn-compiler for second review. Thanks

[jcouv]

@dotnet/roslyn-compiler for second review. Thanks