LLVMTargetMachine/TargetPassConfig: Simplify handling of start/stop options; NFC

- Make some TargetPassConfig methods that just check whether options have
  been set static.
- Shuffle code in LLVMTargetMachine around so addPassesToGenerateCode
  only deals with TargetPassConfig now (but not with MCContext or the
  creation of MachineModuleInfo)

llvm-svn: 345918