[Coroutines] Remove unused coroutine builtin/intrinsics llvm.coro.param (NFC-ish)

I found that the coroutine intrinsic llvm.coro.param in documentation
(https://llvm.org/docs/Coroutines.html#id101) didn't get used actually
since there isn't lowering codes in LLVM. I also checked the
implementation of libstdc++ and libc++. Both of them didn't use
llvm.coro.param. So I am pretty sure that the llvm.coro.param intrinsic
is unused. I think it would be better t to remove it to avoid possible
misleading understandings.

Note: according to [class.copy.elision]/p1.3, this optimization is
allowed by the C++ language specification. Let's make it someday.

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D115222