Add -fansi-escape-codes option

Some build systems use pipes for stdin/stderr. On nix-ish platforms colored
output can be forced by -fcolor-diagnostics. On Windows this option has
no effect in these cases because LLVM uses the console API (which only
operates on the console buffer) even if a console wrapper capable of
interpreting ANSI escape codes is used.

The -fansi-escape-codes option allows switching from the console API to
ANSI escape codes. It has no effect on other platforms.