This patch removes the clear() member from <string_view>. The
modifier was removed from the TS before it ever landed in the standard.
There is no reason libc++ should be providing this method.
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@302869 91177308-0d34-0410-b5e6-96231b3b80d8