[libc++] add inline for __open's definition in ifstream and ofstream
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).
Differential Revision: https://reviews.llvm.org/D99422