string: Remove always_inline from basic_string::__init

[dexonsmith]

Summary

This is effectively a partial revert of r278356, which started inlining
basic_string::__init. Even if we want to help the compiler along with
an inlinehint, we shouldn't hamstring it by forcing it to inline all the
time.

Libc++ uses always_inline widely as a limit-the-ABI-hack, but since
__init is already on the dylib boundary, it makes no sense here and just
harms the debugging experience at -O0.

[dexonsmith]

Committed in r299290.

libcxx/include/string

template <size_type __a> static
             {return (__s + (__a-1)) & ~(__a-1);}
     enum {__alignment = 16};
     static _LIBCPP_INLINE_VISIBILITY
     size_type __recommend(size_type __s) _NOEXCEPT
         {return (__s < __min_cap ? static_cast<size_type>(__min_cap) :
                  __align_it<sizeof(value_type) < __alignment ?
                             __alignment/sizeof(value_type) : 1 > (__s+1)) - 1;}
 
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline
     void __init(const value_type* __s, size_type __sz, size_type __reserve);
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline
     void __init(const value_type* __s, size_type __sz);
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline
     void __init(size_type __n, value_type __c);
 
     template <class _InputIterator>
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline
     typename enable_if
     <
         __is_exactly_input_iterator<_InputIterator>::value,
         void
     >::type
     __init(_InputIterator __first, _InputIterator __last);
 
     template <class _ForwardIterator>
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline
     typename enable_if
     <
         __is_forward_iterator<_ForwardIterator>::value,
         void
     >::type
     __init(_ForwardIterator __first, _ForwardIterator __last);
 
     void __grow_by(size_type __old_cap, size_type __delta_cap, size_type __old_sz,