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/D22834

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

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@280944 91177308-0d34-0410-b5e6-96231b3b80d8

include/string

@@ -1798,6 +1798,7 @@ basic_string<_CharT, _Traits, _Allocator>::basic_string(initializer_list<value_t
 #endif  // _LIBCPP_HAS_NO_GENERALIZED_INITIALIZERS
 
 template <class _CharT, class _Traits, class _Allocator>
+inline _LIBCPP_INLINE_VISIBILITY
 basic_string<_CharT, _Traits, _Allocator>::~basic_string()
 {
 #if _LIBCPP_DEBUG_LEVEL >= 2