Test plan for "auto-default structs"

[RikkiGibson]

Spec: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-11.0/auto-default-structs.md
Championed proposal: dotnet/csharplang#5737

• Provide a discoverable setting to enable all auto-default warnings #60176
• EnC (implicit initializations should occur after sequence point for opening { in constructor?)

Compiler

• add proposal to dotnet/csharplang (Create auto-default-structs.md csharplang#5964)
• add to compiler test plan
• -langversion
• field initialization:
  • when all fields are explicitly assigned
  • when no fields are assigned (use initobj?)
    • implementation initializes each field individually.
  • when no fields
  • with auto-properties with get only; with set; with init
    • SemanticErrorTests.CS0843ERR_UnassignedThisAutoProperty,
    • FlowDiagnosticTests.AutoPropInitialization4
    • StructConstructorTests.FieldInitializers_08
  • with field-like events
  • when all sub-fields of a field are explicitly assigned
  • when some sub-fields of a field are explicitly assigned
  • when field is conditionally assigned
  • when explicitly calling declared constructor : this(...)
  • when explicitly calling default parameterless constructor : this()
  • when assigning this = default;
  • when assigning this = default; conditionally
  • when assigning to backing field of semi-auto property (i.e. using a field initializer) moved to Test plan for "field keyword in properties" #57012
  • when assigning to property of semi-auto property (i.e. in a constructor) moved to Test plan for "field keyword in properties" #57012
  • when reading field before assigning
• fields assigned where C#10 produces warnings with /warn:5
• record
  • with explicit fields
  • with explicit auto-properties
• disabled-by-default diagnostics:
  • when all fields are explicitly assigned
  • when no fields are assigned
  • when diagnostics are elevated to warnings or errors
• default assigned:
  • to backing fields of auto-properties
  • for field of type parameter type
  • for field of struct type with parameterless constructor
  • for field of empty struct type (no assignment?)
• field initialization when parameters have !! (which runs first?)
• nullable analysis treats auto-initialized fields as nullable
• #pragma warning restore should not enable disabled diagnostics

Productivity

• Add new error codes to UpgradeProject fixer
  • upgrade from warnings?
• Debugger: reposition to first sequence point in constructor (init fields?)