[libc++] add `inline` for __open's definition in ifstream and ofstream

[ldionne]

Summary

When building with gcc on AIX, it seems that gcc does not like the
always_inline without the inline keyword.
So adding the inline keywords in for open in ifstream and ofstream.
That will also make it consistent with open in basic_filebuf
(it seems we added inline there before for gcc build as well).

(cherry picked from commit 52e9d80d5db20f23979e409f958736d130387f9e)

[ldionne]

Associated release blocking bug: http://llvm.org/PR50734

[ldionne]

This has been merged to release/12.x

libcxx/include/fstream

#endif
     basic_filebuf* open(const string& __s, ios_base::openmode __mode);
 
 #if _LIBCPP_STD_VER >= 17 && !defined(_LIBCPP_HAS_NO_FILESYSTEM_LIBRARY)
     _LIBCPP_AVAILABILITY_FILESYSTEM _LIBCPP_INLINE_VISIBILITY
     basic_filebuf* open(const _VSTD_FS::path& __p, ios_base::openmode __mode) {
       return open(__p.c_str(), __mode);
     }
 #endif
-    inline _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY
     basic_filebuf* __open(int __fd, ios_base::openmode __mode);
 #endif
     basic_filebuf* close();
 
     _LIBCPP_INLINE_VISIBILITY
     inline static const char*
     __make_mdstring(ios_base::openmode __mode) _NOEXCEPT;
 
if (__file_ == nullptr)
         } else
           __rt = nullptr;
       }
     }
     return __rt;
 }
 
 template <class _CharT, class _Traits>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 basic_filebuf<_CharT, _Traits>*
 basic_filebuf<_CharT, _Traits>::__open(int __fd, ios_base::openmode __mode) {
   basic_filebuf<_CharT, _Traits>* __rt = nullptr;
   if (__file_ == nullptr) {
     if (const char* __mdstr = __make_mdstring(__mode)) {
       __rt = this;
       __file_ = fdopen(__fd, __mdstr);
       if (__file_) {

 {
     if (__sb_.open(__s, __mode | ios_base::in))
         this->clear();
     else
         this->setstate(ios_base::failbit);
 }
 
 template <class _CharT, class _Traits>
+inline
 void basic_ifstream<_CharT, _Traits>::__open(int __fd,
                                              ios_base::openmode __mode) {
   if (__sb_.__open(__fd, __mode | ios_base::in))
     this->clear();
   else
     this->setstate(ios_base::failbit);
 }
 #endif

 {
     if (__sb_.open(__s, __mode | ios_base::out))
         this->clear();
     else
         this->setstate(ios_base::failbit);
 }
 
 template <class _CharT, class _Traits>
+inline
 void basic_ofstream<_CharT, _Traits>::__open(int __fd,
                                              ios_base::openmode __mode) {
   if (__sb_.__open(__fd, __mode | ios_base::out))
     this->clear();
   else
     this->setstate(ios_base::failbit);
 }
 #endif