[Coroutines][Docs] Add a discussion on the handling of certain parameter attribs

ByVal arguments and Swifterror require special handling in the coroutine
passes. The goal of this section is to provide a description of how these
parameter attributes are handled.

Author: TylerNowicki

llvm/docs/Coroutines.rst

@@ -810,6 +810,28 @@ The LLVM IR for a coroutine using a Coroutine with a custom ABI looks like:
     ret ptr %hdl
   }
 
+Parameter Attributes
+============
+Some parameter attributes, used to communicate additional information about the result or parameters of a function, require special handling.
+
+ByVal
+---------------------------------
+A ByVal parameter on an argument indicates that the pointer parameter should really be passed by value to the function.
+Prior to the coroutine transforms loads and stores to/from the pointer are generated where the value is needed.
+Consequently, a ByVal argument is treated much like an alloca.
+Space is allocated for it on the coroutine frame and the uses of the argument pointer are replaced with a pointer to the coroutine frame.
+
+Swift Error
+---------------------------------
+Clang supports the swiftcall calling convention in many common targets, and a user could call a function that takes a swifterror argument from a C++ coroutine.
+The swifterror parameter attribute exists to model and optimize Swift error handling.
+A swifterror alloca or parameter can only be loaded, stored, or passed as a swifterror call argument, and a swifterror call argument can only be a direct reference to a swifterror alloca or parameter. 
+These rules, not coincidentally, mean that you can always perfectly model the data flow in the alloca, and LLVM CodeGen actually has to do that in order to emit code.
+
+For coroutine lowering the default treatment of allocas breaks those rules — splitting will try to replace the alloca with an entry in the coro frame, which can lead to trying to pass that as a swifterror argument.
+To pass a swifterror argument in a split function, we need to still have the alloca around; but we also potentially need the coro frame slot, since useful data can (in theory) be stored in the swifterror alloca slot across suspensions in the presplit coroutine. 
+When split a coroutine it is consequently necessary to keep both the frame slot as well as the alloca itself and then keep them in sync.
+
 Intrinsics
 ==========