While reading LWG#526, Ion Gaztañaga noticed that libc++ didn't correctly handle list::remove(const value_type &x), if x was an element of the list. Added a test for this, and a fix. Thanks to Ion for the report.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@215210 91177308-0d34-0410-b5e6-96231b3b80d8