Support reading calling convention from modreq or modopt on the return type

[333fred]

Runtime-side tracking of the extensible calling convention proposal. Compiler-side issue: dotnet/roslyn#39865. Will update with a final description of the actual spec when we've settled it.

[Dotnet-GitSync-Bot]

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[jeffschwMSFT]

@AaronRobinsonMSFT

[AaronRobinsonMSFT]

Based on plans in .NET 5, there are no current reasons to read any modopt values. This would change if/when we add CallConvSuppressGCTransition. Instead the goal here would be to ignore/block cases when we do discover an existing modopt that is attempting to change behavior. The goal here would be preventative for when we do add support in a future runtime.

[AaronRobinsonMSFT]

Related #38133