Add more "ref in async" tests

[jjonescz]

Test plan: #72662

All remaining tests from the test plan should be covered:

• ref / ref struct local in iterator method: RefUnsafeInIteratorAndAsyncTests.LangVersion_*
• unsafe block in async method in C#12-13: UnsafeTests.UnsafeBlock_InAsyncMethod
• effect on unsafe context within iterator's body (the breaking change): UnsafeTests.UnsafeContext_LocalFunctionInIterator_BreakingChange_*
• use ref / ref struct local
  • within / across await: RefUnsafeInIteratorAndAsyncTests.Await_*
  • within / across yield return; in
    • regular iterators: RefUnsafeInIteratorAndAsyncTests.YieldReturn_*
    • async iterators: RefUnsafeInIteratorAndAsyncTests.YieldReturn_*_Async
  • across yield break; in
    • regular iterators: RefUnsafeInIteratorAndAsyncTests.YieldBreak_*
    • async iterators: RefUnsafeInIteratorAndAsyncTests.YieldBreak_*_Async
  • within / across async foreach block: CodeGenAwaitForeachTests.TestWithPattern_Ref*
  • yield in nested block: YieldReturn_*_Across_NestedBlock
  • within async lambda / local function: RefLocalsAndReturnsTests.RefLocal_Async_LocalFunction and similar
  • within iterator local function: RefLocalsAndReturnsTests.RefLocal_Iterator_LocalFunction and similar
  • within property / indexer get iterator: RefUnsafeInIteratorAndAsyncTests.*_Indexer
  • error in async / iterator even in unsafe context - I'm not sure what this means
  • await in elements of stackalloc: StackAllocInitializerTests.TestAwait_Span_02
  • async Task<int[]> M() => ... ReadOnlySpan x = await M();: SpanStackSafetyTests.AwaitAssignSpan
  • Buffer2 b = [await a, await b];: InlineArrayTests.Initialization_RefStruct_Await
  • using (refStruct = ...) { await ...; }: CodeGenAwaitUsingTests.RefStruct_*
  • foreach (var _ in refStruct) { await ...; }: CodeGenAwaitForeachTests.TestWithPattern_RefStructEnumerable_Async
  • interpolation handlers - the ref struct is not used when there are awaits - InterpolationTests.AwaitInHoles_UsesFormat
• unsafe context > error for await: BindingAsyncTests.BadAwaitInUnsafeContext

[jjonescz]

@AlekseyTs @cston for reviews, thanks

[AlekseyTs]

// (6,22): error CS4007: Instance of type 'R' cannot be preserved across 'await' or 'yield' boundary.

It feels like it would be better to report this error across all language versions #Resolved

[jjonescz]

Right, I will link the issue in a comment. Thanks.

[AlekseyTs]

ref struct Buffer

I would be good to have a test when this is not a ref struct. The scenario will use references underneath. #Resolved

[AlekseyTs]

yield return y[0];

This looks interesting. Do we understand why this works? The span is handled specially. What component does that and where?
I missed the re-assignment #Closed

[AlekseyTs]

LGTM (commit 1)