This commit implements a proposed addendum to the C++ Coroutines TS that
would allow users to query, at compile time, the maximum potential size
of a coroutine frame, from within the frontend.
The builtin_coro_frame_max_size allows users to access this size, for
coroutine lambdas, by passing in the type of the coroutine lambda into
the function from which builtin_coro_frame_max_size is called.
https://reviews.llvm.org/P8124 provides an example of such a program.