[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 am: f4f52f6f73
am: 367fe25436

Change-Id: I63770bffb468748510e0b20c13ed1f5cc84fe093