Added 'inline' attribute to basic_string's destructor

Author: laxmansole

Reviewers: howard.hinnant
           mclow.lists
Subscribers: EricWF, flyingforyou, evandro

Differential Revision: https://reviews.llvm.org/D25624

Reapplying the patch as the bug https://llvm.org/bugs/show_bug.cgi?id=30341 is fixed.

Currently basic_string's destructor is not getting inlined. So adding 'inline' attribute to ~basic_string().
Worked in collaboration with Aditya Kumar.

llvm-svn: 285456

libcxx/include/string

@@ -806,6 +806,7 @@ public:
     basic_string(initializer_list<value_type> __il, const allocator_type& __a);
 #endif  // _LIBCPP_HAS_NO_GENERALIZED_INITIALIZERS
 
+    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
     ~basic_string();
 
     _LIBCPP_INLINE_VISIBILITY