Adding -fcolor-diagnostics is causing problem to users that parse the build

output. Peter Collingbourne also reports that it is showing up in
$(llvm-config --cflags).

Revert this for now since I don't know enough cmake to fix it properly.

This reverts commit 18efed7adc79c1970f307bb5b015d199012ba872.

llvm-svn: 156392