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

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).

Differential Revision: https://reviews.llvm.org/D99422