Expose a capability API for Reflection Emit

[terrajobst]

Assuming #829 goes in, we should consider adding a a capability API. This allows implementations that cannot support runtime code generation to expose up-front that they will fail at runtime with PlatformNotSupportedException. This allows consumers to write code like this:

if (IsRefEmitAvailable())
{
    EmitAndRun();    
}
else
{
    Interpret();
}

Strictly speaking, this is violating our inclusion principles but it seems the opportunity loss seems high for what amounts to be a simple API.

[morganbr]

It would be great to be able to ask two questions:

1. Does reflection emit work at all on this platform? (Will it just throw PlatformNotSupportedException if I try?)
2. Is reflection emit more performant than regular reflection? (Will it really compile code or will it use a slow interpreter that I'm better off not using?)

[JamesNK]

I think this is needed. Without a flag to test I think library authors would end up in the situation of wrapping IL gen in a try/catch and falling back on exception. Gross.

[terrajobst]

We now have a design for this API. Modulo other feedback form our API review experts, I expect the API to be approved early next week.

https://github.com/dotnet/corefx/issues/29258