basic_string_view's non-member begin/end should take basic_string_vie…

…w by value

... as the working draft requires. Test coverage failed to detect this issue due to an overload resolution bug (https://developercommunity.visualstudio.com/content/problem/739010/overload-resolution-fails-to-select-deleted-overlo.html) in MSVC.

Drive-by: Remove the "accepts rvalues" bit from the comment in `begin` which caused the confusion that gave rise to microsoft#104. Hopefully it is now glaringly obvious that `begin` and `end` accept both lvalues and rvalues.

Resolves microsoft#104.

stl/inc/xstring

@@ -1205,11 +1205,11 @@ public:
     }
 
 #ifdef __cpp_lib_concepts
-    _NODISCARD friend constexpr const_iterator begin(const basic_string_view& _Right) noexcept {
-        // non-member overload that accepts rvalues to model the exposition-only forwarding-range concept
+    _NODISCARD friend constexpr const_iterator begin(basic_string_view _Right) noexcept {
+        // non-member overload to model the exposition-only forwarding-range concept
         return _Right.begin();
     }
-    _NODISCARD friend constexpr const_iterator end(const basic_string_view& _Right) noexcept {
+    _NODISCARD friend constexpr const_iterator end(basic_string_view _Right) noexcept {
         // Ditto modeling forwarding-range
         return _Right.end();
     }