Fix #3908: use the async iterator element type for yield

[sailro]

Fixes #3908.

Problem

StatementBuilder.VisitYieldReturn derives the target element type from IEnumerable<T> /
IEnumerator<T>. For an async iterator, that lookup returns an unknown type even though
AsyncAwaitDecompiler has already recovered the element type.

Without the target type, the compiler-generated box T used to store a constrained generic value
in the async iterator's current field remains visible as (object)value. The resulting
yield return fails with CS0266 when the iterator returns an interface constraint.

Solution

Use ILFunction.AsyncReturnType as the element type for async iterators, falling back to the
existing synchronous enumerable lookup.

The existing expression conversion logic can then remove compiler boxing when an implicit
conversion to the recovered element type exists. No async-state-machine-specific cast rewrite is
needed.

Tests

Extended the AsyncStreams Pretty fixture with:

• a constrained generic async iterator returning its interface constraint;
• synchronous and asynchronous object iterators yielding boxed bool, char, and enum values,
  ensuring their source values remain intact.

Validation:

• AsyncStreams Pretty matrix: 6 passed

• runtime-async AsyncStreams matrix: 2 passed

• full Pretty suite: 1,913 passed, 5 existing skips

• Release solution build: 0 warnings, 0 errors

• full solution test suite: 4,921 passed, 15 existing skips

• At least one test covering the code changed

[siegfriedpammer]

Thank you for your contribution!