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
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@299967
91177308-0d34-0410-b5e6-
96231b3b80d8