[libc++] Rename span's as_writeable_bytes to as_writable_bytes

Summary: The Standard says as_writable_bytes.

Reviewers: mclow.lists, EricWF

Subscribers: christof, jkorous, dexonsmith, libcxx-commits

Differential Revision: https://reviews.llvm.org/D59882

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@357139 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed