[coroutines] Fix co_return statement for initializer list arguments

Summary:
Previously the statement `co_return {42}` would be transformed into `P.return_void()`, since the type of `{42}` is represented as `void` by Clang.

This patch fixes the bug by checking for `InitListExpr` arguments and transforming them accordingly.

Reviewers: majnemer, GorNishanov, rsmith

Subscribers: mehdi_amini, cfe-commits

Differential Revision: https://reviews.llvm.org/D25296

llvm-svn: 283495