[RuntimeAsync] EnC support

[VSadov]

RuntimeAsync will need some form of EnC support.

This could range from forbidding EnC when RuntimeAsync is enabled, to supporting only for Task-returning methods (but not Async), to supporting in all cases.
There could be multiple options to make this work. It is possible that we would need to use less optimal, but more EnC-compatible codegen when EnC is enabled.

The level of support may need to be coordinated with teams who actually provide EnC services.

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[VSadov]

As discussed, this will be a part of a more general diagnostic story, so does not need to be tracked separately.

CC. @tommcdon