[Coroutines] Set presplit attribute in Clang instead of CoroEarly pass

Presplit coroutines cannot be inlined. During AlwaysInliner we check if a function is a presplit coroutine, if so we skip inlining.
The presplit coroutine attributes are set in CoroEarly pass.
However in O0 pipeline, AlwaysInliner runs before CoroEarly, so the attribute isn't set yet and will still inline the coroutine.
This causes Clang to crash: https://bugs.llvm.org/show_bug.cgi?id=49920

To fix this, we set the attributes in the Clang front-end instead of in CoroEarly pass.

Reviewed By: rjmccall, ChuanqiXu

Differential Revision: https://reviews.llvm.org/D100282