Properties with mixed readonly accessors should skip escape checks where possible #35606
Description
In some cases involving mixed readonly/non-readonly properties and indexers, we should be able to tell that passing a ref-like variable to a readonly member will not allow that variable to escape, but we're currently pessimistic and require that all accessors be 'readonly' in order to decide the variable won't escape.
using System;
public ref struct S
{
int this[Span<byte> span] { readonly get => 0; set {} }
static void M(S s)
{
Span<byte> span = stackalloc byte[10];
// We know 'span' won't escape because the getter is readonly
// but we still give an error today.
_ = s[span];
}
}