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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190464 91177308-0d34-0410-b5e6-96231b3b80d8