Make LTO codegen use a PassManager, rather than a FunctionPassManager, for the

codegen passes. This brings it in to line with clang and llc's codegen setup,
and tidies up the code.

If I understand correctly, adding ModulePasses to a FunctionPassManager is
bogus. It only seems to explode if an added ModulePass depends on a
FunctionPass though, which might be why this code has survived so long.

Fixes <rdar://problem/13386816>.

llvm-svn: 176977