are two motivations for this.
First, this allows users who are specializing __gnu_ext::hash to
continue doing so without changing their code.
Second, SGI specifies hash overloads for char* and const char* that
perform a hash of the string, not of the pointer.
In order to support this, the hashing code for string is factored out.
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@136539 91177308-0d34-0410-b5e6-96231b3b80d8
