NullableWalker.Scan().getFieldSymbolToBeInitialized(): Is checking IsAutoProperty sufficient or should we check the appropriate accessors are missing or auto-implemented

[RikkiGibson]

          I will try and resolve this comment in my upcoming PR. I think we should just check for a `BackingField` and return it here.

Originally posted by @RikkiGibson in #75116 (comment)

Code being referenced:

roslyn/src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.cs

Lines 1076 to 1077 in bdebb57

	static Symbol getFieldSymbolToBeInitialized(Symbol requiredMember)
	=> requiredMember is SourcePropertySymbol { IsAutoProperty: true } prop ? prop.BackingField : requiredMember;

It feels like this is another piece of our attempts to correctly substitute the field for the property and vice-versa when initializing/reading/managing flow state, and this check/branch can probably be consolidated into another area. But it doesn't appear to be urgent or impacting correctness.