Test plan for "unmanaged generic structs"

This issue is a place to collect test ideas and track their validation. Use the compiler test plan as a tool for brainstorming.

validate with LangVersion 7.3
test with tuple type (SO thread) Issue with tuples and unmanaged constraint #32103
write speclet (contained under Design section of proposal)
~~[ ] update compiler test plan~~ (doesn't appear to be anything to add)
test public API IsUnmanagedType in new scenarios
Test for ref structs
Test the IsManagedType API as suggested by Julien
Test moveability (existing rules should be sufficient)
Have at least one test spit some IL to look at and have a few tests run with "expected output" to make sure the CLR can handle the code.
Stackalloc array of items
(Mutually) Recursive struct definition
- Also check the IsUnmanagedType API for these
Fixed-size buffer inside generic struct
Also check across metadata (metadata struct definition has private object or unmanaged field)
- Also private T field
For all places unmanaged type is required, we should have a test that covers that code path and ensures that the required feature diagnostic is produced at language version < 8.0
- addressOf &
- fixed with address of field
- fixed with array of generic structs
- sizeof
- stackalloc with size
- implicit stackalloc with initializer
- field of type pointer to unmanaged generic struct
- unmanaged constraint

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