This fixes an assertion error when writing a coroutine with a
function-try-block. In this case the function body is not a
`CompoundStmt` so the code constructing an artificial CXXTryStmt must
also construct a `CompoundStmt` for it.
While on it adjust the `CXXStmt::Create` function to only accept
`CompoundStmt*`.
Differential Revision: https://reviews.llvm.org/D146758
