[automerger skipped] DO NOT MERGE: [libc++] Add `__truncating_cast` for safely casting float types to integers am: 55b6136b3a am: 9546d85a3d am: c3a80bd37a -s ours am: f126b4ea11 -s ours
am: 9432e1044b

Change-Id: I119d98960f63fafb4f15c141b29dbc92d13cfd5b