Disable use-color if the output stream is not a terminal with color support.

Summary:
LLDB currently only checks the output terminal for color support by looking
at the `TERM` environment variable and comparing it to `"dumb"`. This causes that
when running LLDB on a CI node, the syntax highlighter will not be deactivated by
LLDB and the output log is filled with color codes (unless the terminal emulator
actually exposes itself as dumb).

This patch now relies on the LLVM code for detecting color support which is more
reliable. We now also correctly actually initialize the `m_supports_colors` variable in `File`.
`m_supports_colors` was so far uninitialized, but the code path that uses `m_supports_colors`
was also dead so the sanitizers didn't sound an alarm.

The old check that compares `TERM` is not removed by this patch as the new LLVM code
doesn't seem to handle this case (and it's a good thing to check for "dumb" terminals).

Reviewers: aprantl, javed.absar

Reviewed By: aprantl

Subscribers: kristof.beyls, abidh, lldb-commits

Differential Revision: https://reviews.llvm.org/D51243

llvm-svn: 340747