Allow incomplete template types in unique_function arguments

We can't declare unique_function that has in its arguments a reference to
a template type with an incomplete argument.
For instance, we can't declare unique_function<void(SmallVectorImpl<A>&)>
when A is forward declared.

This is because SFINAE will trigger a hard error in this case, when instantiating
IsSizeLessThanThresholdT with the incomplete type.

This patch specialize AdjustedParamT for references to remove this error.

Committed on behalf of: @math-fehr (Fehr Mathieu)

Reviewed By: DaniilSuchkov, yrouban