ThinLTO: sort inputs and schedule by decreasing size

This is a compile time optimization: keeping a large file to process
at the end hurts parallelism.
The heurisitic used right now is the input buffer size, however we
may want to consider the number of functions to import or the
different number of files to load for importing as well.

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 269684