MC: Remove the copy of MCSchedModel in MCSubtargetInfo

MC: Remove the copy of MCSchedModel in MCSubtargetInfo

`MCSchedModel` is large.  Make `MCSchedModel::GetDefaultSchedModel()`
return by-reference instead of by-value, so we can store a pointer in
`MCSubtargetInfo::CPUSchedModel` instead of a copy.

Note: since `MCSchedModel` is POD, this doesn't create a static
constructor.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@241947 91177308-0d34-0410-b5e6-96231b3b80d8