Reflection.Emit: Allow `ParameterBuilder.SetConstant(null)` for value-typed parameter...

Reflection.Emit: Allow `ParameterBuilder.SetConstant(null)` for value-typed parameters (#17887)

* Add test for dotnet/corefx#26184

The Roslyn C# compiler encodes an optional, value-typed parameter's
default value of `default(TValueType)` as a null reference constant in
metadata. Add a test that verifies that reflection can do the same
using `ParameterBuilder.SetConstant(null)`.

* Always allow ParameterBuilder.SetConstant(null)

* Remove test project as requested in review

These tests move to CoreFX. See dotnet/corefx#29532.