add a new --print-diagnostic-categories option, which causes the driver to

print out all of the category numbers with their description.  This is useful
for clients that want to map the numbers produced by 
--fdiagnostics-show-category=id to their human readable string form.  The
output is simple but utilitarian:

$ clang --print-diagnostic-categories
1,Format String
2,Something Else

This implements rdar://7928193

llvm-svn: 103080