If a user defines a coroutine_traits type that takes an incorrect
number of template parameters, or for some reason they include such
a type in their program, they receive a cryptic error message:
"too few template arguments for class template 'coroutine_traits'".
The problem is that they may not understand how many is the right
number of template arguments.
Add a note diagnostic that explains why the coroutine_traits template
is being instantiated, and with what arguments.
Test Plan: check-clang