switch Warnings.cpp to use the diagnostics machinery to print diagnostics, not *fprintf*!

Among other things, this makes the warning about unknown warning options mappable.
For example:

$ clang t.c -Werror -Wfoo
error: unknown warning option '-Wfoo' [-Wunknown-warning-option]

For another thing, they are properly color coded now too :)

llvm-svn: 73936