Fix std::string assignment ambiguity from braced initializer lists.

When support for `basic_string_view` was added to string it also
added new assignment operators from `basic_string_view`. These caused
ambiguity when assigning from a braced initializer. This patch fixes
that regression by making the basic_string_view assignment operator
rank lower in overload resolution by making it a template.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@292276 91177308-0d34-0410-b5e6-96231b3b80d8

include/string

@@ -818,6 +818,7 @@ public:
     operator __self_view() const _NOEXCEPT { return __self_view(data(), size()); }
 
     basic_string& operator=(const basic_string& __str);
+    template <class = void>
     _LIBCPP_INLINE_VISIBILITY
     basic_string& operator=(__self_view __sv)  {return assign(__sv);}
 #ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES