[libc++] Make SFINAE'd member functions in string mutually exclusive.

This patch is needed in order to work around a GCC bug that fails to
explicitly instantiate a non-template function of a class template when
there is another overload that's a function template.
(See https://godbolt.org/z/4bUQ_b)

This patch SFINAE's away the function templates when the argument is
a basic_string.