[clang] [CIR][docs] Add cir.catch_param to ClangIR cleanup and EH design doc (PR #181284)

[Andy Kaylor via cfe-commits]

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@@ -350,11 +350,22 @@ the destructor. Therefore, this cleanup handler is marked as eh_only.
 Try-catch blocks will be represented, as they are in the ClangIR
 incubator project, using a `cir.try` operation.
 
+The `cir.catch_param` operation is used to represent the capturing of the
+exception object in an ABI-independent way. When the catch handler includes
+a source variable representing the exception object, the result of the
+`cir.catch_param` operation will be stored to an alloca object for the
+source variable.  If the handler is a catch-all, the `cir.catch_param` operation
+will return a pointer to void, but this cannot be captured by a source variable.
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andykaylor wrote:

Near as I can tell, `std::current_exception` is implemented as a library call, and we don't even have a definition of it from the `exception` header. Here's the classic codegen handling:

https://godbolt.org/z/1db9Tz3W7

It may be that we could attach some information to the `std::current_exception` call that would allow for some alias analysis, and possibly if we know the target ABI we could fold the result with the result of `cir.catch_param` with the call in some way, but I'm not prepared to document any such claim right now.

https://github.com/llvm/llvm-project/pull/181284