[Support] Change isatty to is_displayed

Currently, when building with the Unix support library and `isatty` does
not exist for the target platform (i.e. `HAVE_ISATTY` is false),
compilation of the file `raw_ostream.cpp` will fail due to direct use of
`isatty` in the function `raw_fd_ostream::preferred_buffer_size()`.

Use is_displayed() to fix the problem.

Reviewed By: probinson, MaskRay

Differential Revision: https://reviews.llvm.org/D75278