llvm-lto2: Move the LTO::run() action behind a subcommand.

Move LTO::run() to a "run" subcommand so that we can introduce new subcommands
for testing different parts of the LTO implementation.

This doesn't use llvm::cl subcommands because it doesn't appear to be currently
possible to pass an argument not associated with a subcommand to a subcommand
(e.g. -lto-use-new-pm, -mcpu=yonah).

Differential Revision: https://reviews.llvm.org/D31410

llvm-svn: 299967